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/utils/helpers.js

@@ -0,0 +1,33 @@
+/**
+ * Shared Utility Functions
+ *
+ * General-purpose helper functions used across multiple modules.
+ */
+
+/**
+ * Format duration in milliseconds to human-readable string
+ * @param {number} ms - Duration in milliseconds
+ * @returns {string} Human-readable duration (e.g., "1h23m45s")
+ */
+export function formatDuration(ms) {
+    const seconds = Math.floor(ms / 1000);
+    const hours = Math.floor(seconds / 3600);
+    const minutes = Math.floor((seconds % 3600) / 60);
+    const secs = seconds % 60;
+
+    if (hours > 0) {
+        return `${hours}h${minutes}m${secs}s`;
+    } else if (minutes > 0) {
+        return `${minutes}m${secs}s`;
+    }
+    return `${secs}s`;
+}
+
+/**
+ * Sleep for specified milliseconds
+ * @param {number} ms - Duration to sleep in milliseconds
+ * @returns {Promise<void>} Resolves after the specified duration
+ */
+export function sleep(ms) {
+    return new Promise(resolve => setTimeout(resolve, ms));
+}