docs: expand TROUBLESHOOTING.md with common scaffold failures

[TrueFurina]

Summary

Per #272: expands docs/TROUBLESHOOTING.md with short recipes for common scaffold failures.

Changes

• New ## uv not found — symptoms (uvx: command not found), cause (uv not installed / not on PATH), install commands per platform, verification
• New ## Windows file:// URL quirks — drive-letter and forward-slash requirements, set / PowerShell examples, fallback advice
• New ## Homebrew / AUR version lag — why packages lag, upgrade commands for both, and the uvx ...@latest escape hatch
• New ## Still stuck? — Discord link

Existing sections already covered catalog fetch failures (Forcing a fresh fetch), non-empty target dir, and incompatible extension combinations (incompatibleWith errors), so those were left as-is.

[coderabbitai]

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_{Comment @coderabbitai help to get the list of available commands.}

[ulises-jeremias]

Great follow-through on #272, @TrueFurina — reviewed the full diff and the recipes are accurate and match the CLI behavior (--force, CPA_CATALOG_URL, cache fallback). ✅

• uv not found covers the actual failure mode we see from first-time users.
• The Windows file:// drive-letter notes are correct and easy to miss otherwise — good catch.

Merging. Also — we'd love to have you in the community Discord where we coordinate contributions and answer questions: https://discord.gg/bR5VyATgka — hope to see you there! 🙌