docs(upgrade): Claude Code files live in .claude/skills, not .claude/commands (#3708)

The Claude Code integration installs skills into `.claude/skills` (see
integrations/claude: `"dir": ".claude/skills"`), and the "what gets kept"
list earlier in this same doc already says `.claude/skills/`. But three
troubleshooting/reference spots still point users at `.claude/commands/`,
which does not exist for a Claude Code install -- so the "verify files
exist" checks list an empty/missing directory. Correct all three to
`.claude/skills/`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/upgrade.md

@@ -303,7 +303,7 @@ Alternatively, run the `/speckit.specify` command which creates `.specify/featur
 2. **For CLI-based agents**, verify files exist:
 
    ```bash
-   ls -la .claude/commands/      # Claude Code
+   ls -la .claude/skills/        # Claude Code
    ls -la .gemini/commands/      # Gemini
    ls -la .cursor/skills/      # Cursor
    ls -la .pi/prompts/           # Pi Coding Agent
@@ -352,7 +352,7 @@ This warning appears when you run `specify init --here` (or `specify init .`) in
 
 Only Spec Kit infrastructure files:
 
-- Agent command files (`.claude/commands/`, `.github/prompts/`, etc.)
+- Agent command/skill files (`.claude/skills/`, `.github/prompts/`, etc.)
 - Scripts in `.specify/scripts/`
 - Templates in `.specify/templates/`
 - Missing memory files such as `.specify/memory/constitution.md` may be created from templates; an existing constitution is preserved
@@ -441,7 +441,7 @@ Once you've run `specify init`, the slash commands (like `/speckit.specify`, `/s
    ls -la .github/prompts/
 
    # For Claude
-   ls -la .claude/commands/
+   ls -la .claude/skills/
 
    # For Pi
    ls -la .pi/prompts/