docs(graph): document supported syntax and preview limitations

[theDakshJaitly]

Target branch: code-graph-preview

Goal

Give preview testers a concrete support reference that distinguishes implemented behavior, graceful degradation, and intentionally unsupported syntax. Documentation must reflect tested behavior rather than aspirational support.

Scope

• Add docs/code-graph-support.md.
• Document the current TypeScript, TSX, JavaScript, and JSX support matrix.
• List extracted node kinds and relationships with small source examples.
• Document the Express resolver's current route-to-handler behavior.
• Explain unsupported languages, missing grammars, and SQLite fallback behavior.
• Document the Node.js 22.5 requirement for the preview.
• Describe known limitations around dynamic dispatch, ambiguous references, generated code, and framework-specific behavior.
• Link the support page from the README code-graph section and docs/extractors.md.

Acceptance criteria

• Every support claim links to or names a fixture/test that proves it.
• Supported, partially supported, and unsupported behavior are visibly distinct.
• The page states that v0.7.0 is an unreleased developer preview.
• The page does not imply Python, Go, Rust, NestJS, or Next.js support before those changes merge.
• Graceful degradation and unsupported-language behavior are documented.
• Contributor-facing and user-facing documentation link to the new page.
• All local Markdown links resolve.

Out of scope

• Adding or changing extractor behavior.
• Promising release dates or specific future language versions.
• Documenting unmerged starter issues as completed support.

Contributor workflow

git switch code-graph-preview
git pull
git switch -c docs/code-graph-support-matrix

Open the pull request with base branch code-graph-preview.