docs(install): add pipx as alternative installation method (#2288)

* docs(install): add pipx as alternative installation method

- Add pipx commands to README.md installation section
- Add note about pipx compatibility to docs/installation.md
- Mention pipx persistent installation in docs/quickstart.md
- Add pipx upgrade instructions to docs/upgrade.md
- Clarify that project has no uv-specific dependencies

Refs: https://github.com/github/spec-kit/discussions/2255

* docs(install): address Copilot feedback - update prerequisites and upgrade references for pipx

* Update docs/quickstart.md

markdownlint’s MD012 (enabled in this repo) flags multiple consecutive blank lines

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update docs/upgrade.md

In the Quick Reference table, the label “pipx upgrade” is misleading because the command shown is `pipx install --force ...` (a reinstall). by copilot.

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

docs/installation.md

@@ -4,7 +4,7 @@
 
 - **Linux/macOS** (or Windows; PowerShell scripts now supported without WSL)
 - AI coding agent: [Claude Code](https://www.anthropic.com/claude-code), [GitHub Copilot](https://code.visualstudio.com/), [Codebuddy CLI](https://www.codebuddy.ai/cli), [Gemini CLI](https://github.com/google-gemini/gemini-cli), or [Pi Coding Agent](https://pi.dev)
-- [uv](https://docs.astral.sh/uv/) for package management
+- [uv](https://docs.astral.sh/uv/) for package management (recommended) or [pipx](https://pypa.github.io/pipx/) for persistent installation
 - [Python 3.11+](https://www.python.org/downloads/)
 - [Git](https://git-scm.com/downloads)
 
@@ -24,6 +24,13 @@ uvx --from git+https://github.com/github/spec-kit.git@vX.Y.Z specify init <PROJE
 uvx --from git+https://github.com/github/spec-kit.git specify init <PROJECT_NAME>
 ```
 
+> [!NOTE]
+> For a persistent installation, `pipx` works equally well:
+> ```bash
+> pipx install git+https://github.com/github/spec-kit.git@vX.Y.Z
+> ```
+> The project uses a standard `hatchling` build backend and has no uv-specific dependencies.
+
 Or initialize in the current directory:
 
 ```bash

docs/quickstart.md

@@ -22,6 +22,17 @@ uvx --from git+https://github.com/github/spec-kit.git specify init <PROJECT_NAME
 uvx --from git+https://github.com/github/spec-kit.git specify init .
 ```
 
+> [!NOTE]
+> You can also install the CLI persistently with `pipx`:
+> ```bash
+> pipx install git+https://github.com/github/spec-kit.git
+> ```
+> After installing with `pipx`, run `specify` directly instead of `uvx --from ... specify`, for example:
+> ```bash
+> specify init <PROJECT_NAME>
+> specify init .
+> ```
+
 Pick script type explicitly (optional):
 
 ```bash

docs/upgrade.md

@@ -9,6 +9,7 @@
 | What to Upgrade | Command | When to Use |
 |----------------|---------|-------------|
 | **CLI Tool Only** | `uv tool install specify-cli --force --from git+https://github.com/github/spec-kit.git@vX.Y.Z` | Get latest CLI features without touching project files |
+| **CLI Tool Only (pipx)** | `pipx install --force git+https://github.com/github/spec-kit.git@vX.Y.Z` | Reinstall/upgrade a pipx-installed CLI to a specific release |
 | **Project Files** | `specify init --here --force --ai <your-agent>` | Update slash commands, templates, and scripts in your project |
 | **Both** | Run CLI upgrade, then project update | Recommended for major version updates |
 
@@ -34,6 +35,14 @@ Specify the desired release tag:
 uvx --from git+https://github.com/github/spec-kit.git@vX.Y.Z specify init --here --ai copilot
 ```
 
+### If you installed with `pipx`
+
+Upgrade to a specific release:
+
+```bash
+pipx install --force git+https://github.com/github/spec-kit.git@vX.Y.Z
+```
+
 ### Verify the upgrade
 
 ```bash