fix: prevent WebSocket reconnection storm on IM token expiry

[Jerry-Xin]

Closes #135

Changes

socket.ts (3 changes)

1. Stale WS close guard — if (this.ws !== ws) return prevents old WebSocket close events from triggering phantom reconnects
2. Exponential backoff with jitter — replaced fixed 3s delay with min(3000 × 2^attempts, 60000) plus ±25% random jitter
3. Reset backoff on CONNACK success — reconnectAttempts = 0 after successful connection

channel.ts (3 changes)

4. Time-based cooldown — replaced hasRefreshedToken boolean with 60s cooldown (lastTokenRefreshAt + TOKEN_REFRESH_COOLDOWN_MS)
5. Removed reset in onConnected — no longer resets the refresh guard on every successful connect
6. Stagger delay + shutdown guard — random 0-5s delay before reconnect after token refresh, with if (stopped) return to prevent reconnection during shutdown

Tests

• socket.test.ts — 5 new tests (backoff progression, delay cap, stale WS guard, reconnect counter reset)
• channel.test.ts — 5 new tests (cooldown allow/block/expire, failure keeps cooldown, stagger bounds)

All 204 tests pass (194 existing + 10 new).