refactor: centralize constants, add error classes, and DRY test utilities

- Create src/errors.js with custom error classes (RateLimitError, AuthError, ApiError, etc.)
- Create src/utils/helpers.js with shared utilities (formatDuration, sleep)
- Create tests/helpers/http-client.cjs with shared test utilities (~250 lines deduped)
- Centralize OAuth config and other constants in src/constants.js
- Add JSDoc types to all major exported functions
- Refactor all test files to use shared http-client utilities
- Update CLAUDE.md with new architecture documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/token-extractor.js

@@ -1,26 +1,22 @@
 /**
  * Token Extractor Module
  * Extracts OAuth tokens from Antigravity's SQLite database
- * 
+ *
  * The database is automatically updated by Antigravity when tokens refresh,
  * so this approach doesn't require any manual intervention.
  */
 
 import { execSync } from 'child_process';
-import { homedir } from 'os';
-import { join } from 'path';
-import { TOKEN_REFRESH_INTERVAL_MS, ANTIGRAVITY_AUTH_PORT } from './constants.js';
+import {
+    TOKEN_REFRESH_INTERVAL_MS,
+    ANTIGRAVITY_AUTH_PORT,
+    ANTIGRAVITY_DB_PATH
+} from './constants.js';
 
 // Cache for the extracted token
 let cachedToken = null;
 let tokenExtractedAt = null;
 
-// Antigravity's SQLite database path
-const ANTIGRAVITY_DB_PATH = join(
-    homedir(),
-    'Library/Application Support/Antigravity/User/globalStorage/state.vscdb'
-);
-
 /**
  * Extract token from Antigravity's SQLite database
  * This is the preferred method as the DB is auto-updated