docs: replace deprecated --ai flag with --integration in all documentation (#2359)

* docs: replace deprecated --ai flag with --integration in all documentation

Replace all user-facing --ai, --ai-skills, and --ai-commands-dir references
with their modern equivalents:

- --ai <agent> → --integration <agent>
- --ai-skills → --integration-options="--skills"
- --ai-commands-dir <dir> → --integration generic --integration-options="--commands-dir <dir>"

Updated files:
- README.md (~17 occurrences)
- docs/installation.md (~8 occurrences)
- docs/upgrade.md (~11 occurrences)
- docs/local-development.md (~5 occurrences)
- CONTRIBUTING.md (1 occurrence)
- extensions/EXTENSION-USER-GUIDE.md (1 occurrence)
- src/specify_cli/__init__.py (docstring examples and error messages)

Left unchanged:
- CHANGELOG.md (historical record)
- Test files (intentionally exercise deprecated flag path)
- CLI flag implementation (backward compatibility)

Closes #2358

* docs: address review feedback on pre-existing issues

- Fix duplicate copilot example in README.md (replace with codex)
- Fix invalid gemini --integration-options="--skills" example (gemini
  does not support --skills)
- Update generic integration comment from 'Unsupported agent' to
  'Bring your own agent; requires --commands-dir'
- Clarify EXTENSION-USER-GUIDE.md: skills auto-register for
  skills-based integrations, not only with --integration-options

* docs: replace bare 'AI agent' / 'AI assistant' with 'coding agent' throughout

Full sweep across all documentation and user-facing CLI messages to
align terminology. Bare references like 'AI agent', 'AI assistant',
and 'AI Agent' are replaced with 'coding agent' or 'coding agent
integration' as appropriate.

Intentionally left unchanged:
- 'AI coding agent' (already correct expanded form)
- Deprecated --ai flag help text and error messages (describes the
  deprecated flag itself)
- Community extension descriptions (external project text)
- 'generated by an AI' in CONTRIBUTING.md (general AI, not agent)

* docs: address review — remove deprecated --offline, qualify --skills scope

- Remove --offline from docstring examples (deprecated no-op)
- Remove --offline from CONTRIBUTING.md testing example
- Replace --offline instructions in docs/installation.md with note that
  bundled assets are used by default
- Qualify --integration-options="--skills" in README.md to note it only
  applies to integrations that support skills mode

README.md

@@ -81,9 +81,9 @@ And use the tool directly:
 specify init <PROJECT_NAME>
 
 # Or initialize in existing project
-specify init . --ai copilot
+specify init . --integration copilot
 # or
-specify init --here --ai copilot
+specify init --here --integration copilot
 
 # Check installed tools
 specify check
@@ -105,9 +105,9 @@ Run directly without installing:
 uvx --from git+https://github.com/github/spec-kit.git@vX.Y.Z specify init <PROJECT_NAME>
 
 # Or initialize in existing project
-uvx --from git+https://github.com/github/spec-kit.git@vX.Y.Z specify init . --ai copilot
+uvx --from git+https://github.com/github/spec-kit.git@vX.Y.Z specify init . --integration copilot
 # or
-uvx --from git+https://github.com/github/spec-kit.git@vX.Y.Z specify init --here --ai copilot
+uvx --from git+https://github.com/github/spec-kit.git@vX.Y.Z specify init --here --integration copilot
 ```
 
 **Benefits of persistent installation:**
@@ -123,7 +123,7 @@ If your environment blocks access to PyPI or GitHub, see the [Enterprise / Air-G
 
 ### 2. Establish project principles
 
-Launch your AI assistant in the project directory. Most agents expose spec-kit as `/speckit.*` slash commands; Codex CLI in skills mode uses `$speckit-*` instead.
+Launch your coding agent in the project directory. Most agents expose spec-kit as `/speckit.*` slash commands; Codex CLI in skills mode uses `$speckit-*` instead.
 
 Use the **`/speckit.constitution`** command to create your project's governing principles and development guidelines that will guide all subsequent development.
 
@@ -302,7 +302,7 @@ Run `specify integration list` to see all available integrations in your install
 
 ## Available Slash Commands
 
-After running `specify init`, your AI coding agent will have access to these slash commands for structured development. If you pass `--ai <agent> --ai-skills`, Spec Kit installs agent skills instead of slash-command prompt files; `--ai-skills` requires `--ai`.
+After running `specify init`, your AI coding agent will have access to these slash commands for structured development. For integrations that support skills mode, passing `--integration <agent> --integration-options="--skills"` installs agent skills instead of slash-command prompt files.
 
 #### Core Commands
 
@@ -475,37 +475,37 @@ specify init --here --force
 
 ![Specify CLI bootstrapping a new project in the terminal](./media/specify_cli.gif)
 
-You will be prompted to select the AI agent you are using. You can also proactively specify it directly in the terminal:
+You will be prompted to select the coding agent integration you are using. You can also proactively specify it directly in the terminal:
 
 ```bash
-specify init <project_name> --ai copilot
-specify init <project_name> --ai gemini
-specify init <project_name> --ai copilot
+specify init <project_name> --integration copilot
+specify init <project_name> --integration gemini
+specify init <project_name> --integration codex
 
 # Or in current directory:
-specify init . --ai copilot
-specify init . --ai codex --ai-skills
+specify init . --integration copilot
+specify init . --integration codex --integration-options="--skills"
 
 # or use --here flag
-specify init --here --ai copilot
-specify init --here --ai codex --ai-skills
+specify init --here --integration copilot
+specify init --here --integration codex --integration-options="--skills"
 
 # Force merge into a non-empty current directory
-specify init . --force --ai copilot
+specify init . --force --integration copilot
 
 # or
-specify init --here --force --ai copilot
+specify init --here --force --integration copilot
 ```
 
 The CLI will check if you have Claude Code, Gemini CLI, Cursor CLI, Qwen CLI, opencode, Codex CLI, Qoder CLI, Tabnine CLI, Kiro CLI, Pi, Forge, Goose, or Mistral Vibe installed. If you do not, or you prefer to get the templates without checking for the right tools, use `--ignore-agent-tools` with your command:
 
 ```bash
-specify init <project_name> --ai copilot --ignore-agent-tools
+specify init <project_name> --integration copilot --ignore-agent-tools
 ```
 
 ### **STEP 1:** Establish project principles
 
-Go to the project folder and run your AI agent. In our example, we're using `claude`.
+Go to the project folder and run your coding agent. In our example, we're using `claude`.
 
 ![Bootstrapping Claude Code environment](./media/bootstrap-claude-code.gif)
 
@@ -517,7 +517,7 @@ The first step should be establishing your project's governing principles using
 /speckit.constitution Create principles focused on code quality, testing standards, user experience consistency, and performance requirements. Include governance for how these principles should guide technical decisions and implementation choices.
 ```
 
-This step creates or updates the `.specify/memory/constitution.md` file with your project's foundational guidelines that the AI agent will reference during specification, planning, and implementation phases.
+This step creates or updates the `.specify/memory/constitution.md` file with your project's foundational guidelines that the coding agent will reference during specification, planning, and implementation phases.
 
 ### **STEP 2:** Create project specifications
 
@@ -725,9 +725,9 @@ The `/speckit.implement` command will:
 - Provide progress updates and handle errors appropriately
 
 > [!IMPORTANT]
-> The AI agent will execute local CLI commands (such as `dotnet`, `npm`, etc.) - make sure you have the required tools installed on your machine.
+> The coding agent will execute local CLI commands (such as `dotnet`, `npm`, etc.) - make sure you have the required tools installed on your machine.
 
-Once the implementation is complete, test the application and resolve any runtime errors that may not be visible in CLI logs (e.g., browser console errors). You can copy and paste such errors back to your AI agent for resolution.
+Once the implementation is complete, test the application and resolve any runtime errors that may not be visible in CLI logs (e.g., browser console errors). You can copy and paste such errors back to your coding agent for resolution.
 
 </details>