[Epic 1] Foundation: Shared Primitives

[edelauna]

Part of #355.

No behavior change. All stories are additive and parallelizable across engineers.

Stories

• Story 1.1 — Extract RateLimitClock singleton
• Story 1.2 — Add TaskSemaphore utility
• Story 1.3 — Register PARALLEL_TOOL_EXECUTION experiment flag

Dependencies

None — can be worked in parallel with Epic 2.

[edelauna]

All three stories delivered:

• Story 1.1 — RateLimitClock extracted to src/core/task/RateLimitClock.ts
• Story 1.2 — TaskSemaphore added at src/utils/TaskSemaphore.ts
• Story 1.3 — PARALLEL_TOOL_EXECUTION experiment flag registered in src/shared/experiments.ts (default off, hidden from settings until parallel dispatch lands)

Epic 3 dependency is satisfied — Story 3.2a (#367) can start.