feat: add local development install and watch scripts

[bestony]

Summary

• Added local developer CLI uninstall flow and made global install/uninstall symmetric:
  • New script: scripts/uninstall-first-tree-global.mjs
  • New npm scripts:
    • cli:uninstall-global -> node scripts/uninstall-first-tree-global.mjs
    • first-tree:uninstall -> pnpm run cli:uninstall-global
    • first-tree:setup / cli:install-global
    • first-tree:dev / cli:watch-build
• CONTRIBUTING.md now documents install + uninstall local dev flows and includes post-uninstall verification (first- tree --version, ft --version).

Why does it matter?

• Makes local global install reversible in a safe, idempotent way and reduces CLI
  development environment drift.
• Gives maintainers a clear cleanup path for global bin entries (first-tree, ft) after local linking during
  development.
• Improves onboarding clarity by documenting both setup and teardown flows in one section.

Compared to dev:

• Commit coverage in this PR range: d615310, b33577f (adds install/watch docs/scripts + uninstall counterpart).
• No CLI runtime/help-output behavior changes.
• (Note) Current local branch is behind origin/dev; directly comparing to origin/dev also shows an additional
  workflow diff (.github/workflows/publish-nightly.yml) from the newer remote tip.