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>

site/src/content/docs/getting-started/configuration.md

@@ -1,9 +1,9 @@
 ---
 title: Configuration
-description: CodeGraph is zero-config — there are no config files.
+description: CodeGraph is zero-config by default, with one optional file for mapping custom extensions.
 ---
 
-There isn't any — CodeGraph is **zero-config**, with **no config file** to write or keep in sync. Language support is automatic from the file extension; there's nothing to wire up per language.
+Next to none — CodeGraph is **zero-config by default**, with nothing to write or keep in sync to get started. Language support is automatic from the file extension; there's nothing to wire up per language. The one optional file is for mapping [custom file extensions](#custom-file-extensions).
 
 ## What it skips out of the box
 
@@ -17,6 +17,23 @@ To keep something else out, add it to `.gitignore`. To pull a default-excluded d
 
 The defaults apply uniformly, so committing a dependency or build directory doesn't force it into the graph — the `.gitignore` negation is the explicit opt-in.
 
+## Custom file extensions
+
+If your project uses a non-standard extension for a [supported language](/codegraph/reference/languages/) — say `.dota_lua` for Lua, or `.tpl` for PHP — those files are skipped by default, because the extension isn't one CodeGraph recognizes. Map them with an optional `codegraph.json` at your project root:
+
+```json
+{
+  "extensions": {
+    ".dota_lua": "lua",
+    ".tpl": "php"
+  }
+}
+```
+
+Each value is a supported language id. The mappings merge on top of the built-in defaults and win on conflict, so you can also re-point a built-in (e.g. `".h": "cpp"`). Commit the file to share the mapping with your team.
+
+A typo'd language or a malformed file is warned about and skipped — it never breaks indexing — and a project with no `codegraph.json` behaves exactly as before. Re-index (`codegraph index`) after adding or changing mappings.
+
 ## Where data lives
 
 Per-project data lives in a `.codegraph/` directory at your project root, containing the SQLite database (`codegraph.db`). Nothing leaves your machine.