perf(subagent): overlap session store initialization

[TheGreatAxios]

Summary

• Start each leaf session store while independent workdir and agent setup proceeds
• Keep session stores scoped to their originating worker without pooling
• Prove the overlap with blocked promises rather than sleeps or filesystem polling

Verification

• bun test src/subagent/run-audit-store.test.ts passes: 3 tests, 0 failures
• bun run typecheck passes
• bun run check passes: lint, typecheck, dead-export guard, build, and 8,597 tests across 551 files
• The overlap test fails the old sequential ordering at expect(storeStarted).toBe(true) while mkdir remains unresolved, then verifies both mkdir and store resolution are required before agent construction
• Benchmark source: scripts/measure-subagent-cold-start.ts
• Benchmark command: bun scripts/measure-subagent-cold-start.ts --samples=7
• Boundaries: a = assemble inference dependencies; b = discover skills and construct skill tools; c = make the workdir and initialize session stores; readiness = a + b + c immediately before agent construction
• The benchmark runs one excluded warmup per mode and alternates measured mode order. Sequential models the parent/base ordering (mkdir, then stores); overlap models this branch (start stores, mkdir, then await stores).

Raw samples in milliseconds:

N	sequential a/b/c/readiness	overlap a/b/c/readiness	paired savings
1	0.025 / 0.631 / 3.610 / 4.267	0.030 / 0.809 / 3.655 / 4.494	-0.227
2	0.020 / 0.770 / 2.616 / 3.407	0.031 / 0.680 / 2.800 / 3.512	-0.105
3	0.022 / 0.599 / 2.493 / 3.114	0.019 / 0.625 / 2.494 / 3.139	-0.025
4	0.020 / 0.613 / 3.907 / 4.540	0.019 / 0.586 / 2.632 / 3.238	1.302
5	0.021 / 0.681 / 4.072 / 4.775	0.037 / 0.664 / 2.489 / 3.191	1.584
6	0.019 / 0.581 / 2.659 / 3.260	0.023 / 0.579 / 2.208 / 2.811	0.450
7	0.044 / 0.835 / 2.804 / 3.684	0.024 / 0.662 / 2.688 / 3.376	0.309

• Before sequential readiness: 3.684 ms median, 3.864 ms mean
• After overlap readiness: 3.238 ms median, 3.394 ms mean
• Realized paired savings: 0.309 ms median, 0.470 ms mean
• Residual c latency on the overlap critical path: 2.632 ms median, 2.710 ms mean

Fixes CL-9010

[linear-code]

CL-9010