docs: distribute from GitHub only (drop PyPI) + SEO + unify roadmap m…

[denfry]

…ilestones

Switch distribution to GitHub-only and stop publishing to PyPI:

• requirements.lock pins codebase-index to the GitHub release tarball (@v1.0.2), so the plugin bootstrap installs from GitHub, not PyPI (no git required).
• release.yml: drop the PyPI trusted-publishing job and id-token permission; tagged GitHub releases with build artifacts are the sole channel.
• README/QUICKSTART/INSTALLATION/COMPARISON/SEO: install via pipx/pip "git+https://[email protected]"; note the package is not on PyPI.
• test_plugin_manifest: assert the lock pins the GitHub tag tarball and never falls back to a PyPI == specifier.

Docs SEO/AEO pass:

• FAQ: definitive opening sentence + "How do I install" Q&A; correct the stale "production-ready" milestone status to reflect shipped v1.0.2.
• QUICKSTART: definitive opening sentence.
• CONTRIBUTING: fix the broken clone URL.

Unify milestone numbering across both roadmaps to the detailed scheme already used by CHANGELOG/LANGUAGES (M0-M9 + M7.5); add MCP bridge as planned M10.

Description

Describe your changes and their impact.

Fixes #(issue)

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional change)
• CI / build / tooling change

Checklist

• I have read the CONTRIBUTING.md guide
• My code follows the project's style conventions (ruff check, ruff format)
• I have added tests that prove my fix is effective or that my feature works
• All tests pass (pytest)
• Type checking passes (mypy, if applicable)
• I have updated the documentation accordingly
• I have updated CHANGELOG.md under [Unreleased]
• My commits follow Conventional Commits

Testing

Describe how you tested your changes:

# Example:
pytest tests/test_my_new_feature.py
codebase-index index --root tests/fixtures/sample_repo
codebase-index search "test query"

Screenshots (if applicable)