feat(directory): CODEGRAPH_DIR env var to override the index dir name (#636) (#741)

Two environments that share one working tree — most concretely Windows
and WSL — can't safely share a single `.codegraph/`: the daemon lockfile
records a platform-specific pid + socket (named pipe vs Unix socket), and
SQLite locking across the WSL2/Windows filesystem boundary is unreliable,
so two daemons over one index risks corruption.

Add a `CODEGRAPH_DIR` env var (default `.codegraph`) that overrides the
per-project data directory name, so each environment keeps its own index
in the same tree (e.g. `CODEGRAPH_DIR=.codegraph-win` on Windows). The
name is resolved live and validated (rejects separators / `..` / absolute,
falling back to the default with a one-time stderr warning). Indexing and
file-watching now skip ANY `.codegraph-*` sibling so neither side trips
over the other's data.

Routes the previously-hardcoded `.codegraph` literals (db path, lockfile,
error log, watcher ignore, file-scan skip, installer) through the
resolver. No extraction-version bump — index content is unchanged.

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

src/db/index.ts

@@ -9,6 +9,7 @@ import * as fs from 'fs';
 import * as path from 'path';
 import { SchemaVersion } from '../types';
 import { runMigrations, getCurrentVersion, CURRENT_SCHEMA_VERSION } from './migrations';
+import { getCodeGraphDir } from '../directory';
 
 export { SqliteDatabase, SqliteBackend } from './sqlite-adapter';
 
@@ -240,5 +241,5 @@ export const DATABASE_FILENAME = 'codegraph.db';
  * Get the default database path for a project
  */
 export function getDatabasePath(projectRoot: string): string {
-  return path.join(projectRoot, '.codegraph', DATABASE_FILENAME);
+  return path.join(getCodeGraphDir(projectRoot), DATABASE_FILENAME);
 }