#22131: fix: clear seqByRun entries in clearAgentRunContext to prevent memory leak

## Summary - `seqByRun` Map in `src/infra/agent-events.ts` grows unbounded because `clearAgentRunContext()` only deletes from `runContextById` but never from `seqByRun` - Every agent run adds an entry via `emitAgentEvent()` that is never cleaned up, causing the gateway process to leak memory indefinitely - Observed 65GB RSS over ~12 hours of active Discord/Slack messaging, with `DiscordMessageListener` taking 120-946 seconds per `MESSAGE_CREATE` due to GC pressure - Fix: add `seqByRun.delete(runId)` to `clearAgentRunContext()` ## Test plan - [x] Verified all 3 call sites (`server-chat.ts` x2, `commands/agent.ts`) only invoke `clearAgentRunContext` at run end/error — no risk of deleting a counter mid-run - [x] Confirmed existing tests pass (`agent-events.test.ts`) - [x] Deployed patched build to production gateway; memory stable at ~600MB after 15+ minutes vs 1.1GB+ and climbing on unpatched version - [x] Reviewed by OpenAI Codex — no correctness or maintainability issues found <!-- greptile_comment --> <h3>Greptile Summary</h3> Fixed critical memory leak where `seqByRun` Map grew unbounded because `clearAgentRunContext()` only deleted from `runContextById` **Key changes:** - Added `seqByRun.delete(runId)` to `clearAgentRunContext()` to properly clean up sequence counters when agent runs complete - Production deployment confirmed memory stabilized at ~600MB vs 1.1GB+ and climbing on unpatched version **Issue found:** - `resetAgentRunContextForTest()` should also clear `seqByRun` to prevent test pollution between test runs <h3>Confidence Score: 4/5</h3> - This PR is safe to merge - it fixes a critical memory leak with a simple, targeted change - The fix correctly addresses the memory leak by ensuring `seqByRun` entries are deleted alongside `runContextById` entries. All three call sites only invoke `clearAgentRunContext()` at run end/error, so there's no risk of mid-run deletion. Production deployment confirms the fix works. Score is 4 instead of 5 because `resetAgentRunContextForTest()` should also clear `seqByRun` for complete test isolation. - src/infra/agent-events.ts needs a minor update to `resetAgentRunContextForTest()` _{Last reviewed commit: bb9f387} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->