Add unit tests for lib/extract-subagent-ids.ts

[NiveditJain]

lib/extract-subagent-ids.ts exports an extractSubagentIds(fileContent: string): string[] that scans newline-delimited JSON for toolUseResult.agentId values on user-type entries, deduplicates them, and returns the result. It has no unit test.

1. Create __tests__/lib/extract-subagent-ids.test.ts — model it on __tests__/lib/format-duration.test.ts.
2. Import: import { extractSubagentIds } from "@/lib/extract-subagent-ids";
3. Cover: (a) happy path with two valid user entries → both ids returned and deduped, (b) malformed JSON line is skipped silently, (c) type !== "user" entries are ignored, (d) non-hex agentId strings are rejected (regex /^[a-f0-9]+$/), (e) missing toolUseResult is ignored, (f) empty input returns [].
4. Run bun run test:run and confirm all cases pass.

Out of scope: changing extract-subagent-ids.ts itself.

[nightt5879]

Opened draft PR #330 for this: #330

Validation: targeted extractSubagentIds test passes (6/6). Full \�un run test:run\ was attempted after installing portable Bun 1.3.13 in the task workspace, but existing Windows path/environment-sensitive tests fail outside this change.