Add Ruby extraction via Prism, not tree-sitter

[emmahyde]

What

Adds a Ruby language extractor so mex graph indexes .rb files. Ruby was previously silently skipped — no grammar, no extractor — so mex graph query where-defined <RailsSymbol> returned TARGET_NOT_FOUND for every Ruby symbol on a real Rails codebase.

Why Prism, not tree-sitter

Rather than vendor the community tree-sitter-ruby grammar (the pattern every other language in this repo follows), this uses @ruby/prism — Ruby core's own parser, the one CRuby 3.3+ ships. It's more semantically accurate for Ruby than a third-party tree-sitter grammar, and it's maintained by the Ruby core team.

Prism's typed AST has no tree-sitter node shape to adapt into, so this needed two small, deliberately isolated deviations from the standard extractor path (documented in docs/extractors.md under "Vendored Grammars > Ruby"):

• src/graph/extraction/prism-runtime.ts — Ruby's parallel loader to grammars.ts's tree-sitter loader (Prism has no relationship to web-tree-sitter).
• rubyExtractor.extract() ignores the passed tree: TSTree parameter entirely and re-parses source with Prism directly. grammars.ts#parse() still returns a placeholder TSTree for Ruby so extractFile's generic if (!tree) return null gate behaves identically across every language — no changes to the frozen LanguageExtractor interface, node identity scheme, or SQLite schema.

Coverage

rubyExtractor (src/graph/extraction/languages/ruby.ts) covers: modules, classes (with superclass extends edges), instance and singleton (def self.x) methods, module/class-scoped constants, mixins (include/extend/prepend → implements), require/require_relative → imports, .new → instantiates, and general method calls.

Fixture (src/graph/__tests__/fixtures/sample.rb) and focused tests (src/graph/__tests__/extractor-ruby.test.ts, 7 tests) follow the existing extractor.test.ts/sample.ts pattern.

Verification

• Verified against real production Rails code (STI phase base class, service objects) with dense method bodies and class-method definitions — zero crashes.
• npx tsc --noEmit — clean.
• npx vitest run — full suite passes except one pre-existing, unrelated failure in test/tui.test.ts (confirmed via git stash/git stash pop bisection: identical failure with zero of this PR's changes present).
• npm run build — clean, .wasm/schema assets copy as expected.
• Built a real graph against a ~1600-file Rails codebase (1681 files, 31,816 nodes, 63,637 edges) and confirmed where-defined/who-calls/graph scope now resolve Ruby symbols end-to-end, including cross-file call chains and STI overrides.

Docs

Updated docs/extractors.md (Vendored Grammars > Ruby, with an explicit deviation note) and docs/code-graph-support.md (added the Ruby row to the support table).

[emmahyde]

Split out a related but independent fix into #117 (graph scope's exact-match scoring was picking up English stopwords from task phrasing — found while building the graph for this Ruby extractor and comparing it against grep, see #115 for the full writeup). No overlap with this PR's changes.