fix: respect env vars and settings.json for DATA_DIR resolution (#1344)

SettingsDefaultsManager.get() returned only the hardcoded default,
ignoring both environment variables and settings.json overrides. This
meant CLAUDE_MEM_DATA_DIR set via env or settings file had no effect
on paths.ts (and other callers using get()).

Two changes:
- get() now checks process.env before falling back to the default
- paths.ts resolves DATA_DIR with full priority: env var > settings.json
  at the default location > hardcoded default

The settings file read uses a synchronous bootstrap pattern to avoid
circular dependencies (DATA_DIR is needed to locate the settings file,
so we check the default path only).

Fixes #1303

Signed-off-by: umut-polat <52835619+umut-polat@users.noreply.github.com>

src/shared/SettingsDefaultsManager.ts

@@ -133,10 +133,15 @@ export class SettingsDefaultsManager {
   }
 
   /**
-   * Get a default value from defaults (no environment variable override)
+   * Get a setting value with environment variable override.
+   * Priority: process.env > hardcoded default
+   *
+   * For full priority (env > settings file > default), use loadFromFile().
+   * This method is safe to call at module-load time (no file I/O) and still
+   * respects environment variable overrides that were previously ignored.
    */
   static get(key: keyof SettingsDefaults): string {
-    return this.DEFAULTS[key];
+    return process.env[key] ?? this.DEFAULTS[key];
   }
 
   /**