Add extractSubagentIds unit tests

[nightt5879]

Closes #327

Summary

• Adds __tests__/lib/extract-subagent-ids.test.ts for extractSubagentIds.
• Covers valid user entries with deduping, malformed JSON, non-user entries, non-hex agent IDs, missing toolUseResult, and empty input.

Verification

• bun run test:run failed initially because bun was not available on PATH in this Windows environment.
• C:\Users\14692\Documents\Codex\2026-05-09\pr-3-agent-pr-issue-10\work\.tools\bun-windows-x64\bun.exe install --frozen-lockfile passed after installing portable Bun 1.3.13 under the task workspace.
• C:\Users\14692\Documents\Codex\2026-05-09\pr-3-agent-pr-issue-10\work\.tools\bun-windows-x64\bun.exe run test:run failed on existing Windows path/environment-sensitive tests: 10 failed files, 65 failed tests, 62 passed files, 1519 passed tests.
• C:\Users\14692\Documents\Codex\2026-05-09\pr-3-agent-pr-issue-10\work\.tools\bun-windows-x64\bun.exe run test:run __tests__/lib/extract-subagent-ids.test.ts passed: 1 file, 6 tests.

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