fix: unbounded maps and memory leaks — Issue #357

[OneStepAt4time]

Fixes #357

Changes (7 files, +194/-61)

1. Periodic prune of ipRateLimits map entries with expired timestamps
2. Delete eventBuffers entry in emitEnded alongside emitter
3. MetricsCollector.cleanupSession() on session kill
4. Cache parsed JSONL entries — read delta only from byteOffset
5. Debounced save for readMessages (5s coalesce, immediate on kill)
6. Replace fixed sleeps with pollUntil in tmux
7. Window existence cache with 2s TTL
8. Set for activeSubagents (O(1) has/delete)
9. Decrement sseTokenCounts when consuming SSE token

1176 tests passing.