#13889: feat: Slack channel cache, session cost alerts & checkpoint/recovery system

## Summary Comprehensive foundational implementation adding three core systems: ### 1. Slack Channel Name Resolution Cache (`src/slack/channel-cache.ts`) - TTL-based in-memory cache for channel name ↔ ID lookups - Forward (name→ID) and reverse (ID→name) lookups with case-insensitive matching - LRU eviction at configurable capacity, bulk loading via `setMany()` - Automatic expiry with `prune()` and per-entry TTL validation - Singleton default instance with `getDefaultChannelCache()` / `resetDefaultChannelCache()` ### 2. Session Cost Alert Thresholds (`src/infra/session-cost-alerts.ts`) - Configurable warning/critical thresholds on both cost (USD) and token count - Per-session deduplication — each threshold fires at most once per session - Async `onAlert` callback for plugging into notification systems (Slack, logs, etc.) - Enable/disable toggle, per-session reset, and global reset - Sensible defaults: warning at $0.50, critical at $2.00 ### 3. Session Checkpoint/Recovery (`src/infra/session-checkpoint.ts`) - Periodic filesystem-based session state checkpointing - Stores transcript length, active model/provider, cost, labels, pending tool calls - Automatic pruning of old checkpoints (configurable retention, default 3) - Minimum interval throttling to prevent excessive writes (default 30s) - `findRecoverableSessions()` for startup recovery discovery - Safe filesystem naming with session ID sanitization ## Testing All three systems include comprehensive test suites: - `src/slack/channel-cache.test.ts` — 10 tests covering CRUD, TTL, eviction, bulk ops - `src/infra/session-cost-alerts.test.ts` — 11 tests covering thresholds, dedup, callbacks, token triggers - `src/infra/session-checkpoint.test.ts` — 8 tests covering save/load, pruning, recovery, throttling ## Motivation These systems form foundational infrastructure for reliable, cost-aware, and resilient agent operation. They share common session lifecycle patterns and were developed together to ensure consistent design. ## Breaking Changes None. All features are additive — new files only, no modifications to existing code. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds three new infra utilities: - `src/slack/channel-cache.ts`: in-memory TTL cache for Slack channel ID/name lookups with a default singleton. - `src/infra/session-cost-alerts.ts`: per-session cost/token threshold monitor with optional callback and default singleton. - `src/infra/session-checkpoint.ts`: filesystem checkpoint store for persisting session state and discovering recoverable sessions, plus unit tests for each module. Overall the changes are additive and well-covered by tests, but there are a few correctness edge cases (stale cache entries, dedupe key collisions, and checkpoint filename collisions) that should be addressed before merging. <h3>Confidence Score: 3/5</h3> - Mostly safe to merge, but there are a few correctness bugs that can cause stale lookups or lost checkpoints under realistic conditions. - The PR is additive and tests cover the happy paths, but there are verified edge-case correctness issues: Slack channel rename can leave stale name mappings, cost alert dedupe can suppress thresholds with distinct token settings, and checkpoint filenames can collide on same-ms saves causing silent data loss. - src/slack/channel-cache.ts; src/infra/session-cost-alerts.ts; src/infra/session-checkpoint.ts <!-- greptile_other_comments_section --> <!-- /greptile_comment -->