feat(config): map custom file extensions to languages via codegraph.json (#906) (#955)

The extension → language table was hardcoded, so a codebase using a
non-standard extension for a supported language (e.g. `.dota_lua` for Lua)
had those files silently skipped — no way to opt them in short of patching
the source.

Add an opt-in, project-scoped `codegraph.json` at the repo root:

    { "extensions": { ".dota_lua": "lua", ".tpl": "php" } }

Mappings merge on top of the built-in defaults and take precedence (so a
built-in can be re-pointed, e.g. `.h` → `cpp`). Absent or malformed config
is the zero-config default — byte-identical to prior behavior; an invalid
target language or unparseable file is warned-and-skipped, never fatal.

Implementation:
- New `src/project-config.ts` — `loadExtensionOverrides(rootDir)`, validated
  against `isLanguageSupported`, mtime-cached per root.
- `detectLanguage` / `isSourceFile` gain an optional `overrides` arg
  (omitting it is the existing behavior).
- Overrides threaded per-operation through every extraction call site
  (scan/walk gates, git change-detection, grammar selection, extraction,
  the file watcher), resolved from the project root — no process-global
  state, so the multi-project daemon stays isolated. The parse worker
  receives the resolved language in its message.

Tests: 13 new cases (unit, loader validation/normalization/caching, and a
full-index integration proving a custom-extension file is extracted while
the zero-config path indexes nothing). Worker path smoke-tested via the
built CLI.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/sync/watcher.ts

@@ -34,6 +34,7 @@
 import * as fs from 'fs';
 import * as path from 'path';
 import { isSourceFile, buildScopeIgnore, type ScopeIgnore } from '../extraction';
+import { loadExtensionOverrides } from '../project-config';
 import { logDebug, logWarn } from '../errors';
 import { normalizePath } from '../utils';
 import { isCodeGraphDataDir } from '../directory';
@@ -535,7 +536,7 @@ export class FileWatcher {
     if (!rel || rel === '.' || rel.startsWith('..')) return;
     if (this.isAlwaysIgnored(rel)) return;
     if (this.ignoreMatcher && this.ignoreMatcher.ignores(rel)) return;
-    if (!isSourceFile(rel)) return;
+    if (!isSourceFile(rel, loadExtensionOverrides(this.projectRoot))) return;
 
     logDebug('File change detected', { file: rel });
     if (this.ready) {