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/bin/codegraph.ts

@@ -356,7 +356,7 @@ function printIndexResult(clack: typeof import('@clack/prompts'), result: IndexR
       clack.log.info(`The index is fully usable ${getGlyphs().dash} only the failed files are missing.`);
     }
   } else if (projectPath) {
-    const logPath = path.join(projectPath, '.codegraph', 'errors.log');
+    const logPath = path.join(getCodeGraphDir(projectPath), 'errors.log');
     if (fs.existsSync(logPath)) {
       fs.unlinkSync(logPath);
     }
@@ -367,7 +367,7 @@ function printIndexResult(clack: typeof import('@clack/prompts'), result: IndexR
  * Write detailed error log to .codegraph/errors.log
  */
 function writeErrorLog(projectPath: string, errors: Array<{ message: string; filePath?: string; severity: string; code?: string }>): void {
-  const cgDir = path.join(projectPath, '.codegraph');
+  const cgDir = getCodeGraphDir(projectPath);
   if (!fs.existsSync(cgDir)) return;
 
   const logPath = path.join(cgDir, 'errors.log');