[Story 1.3] Register PARALLEL_TOOL_EXECUTION experiment flag #363

Part of #356 (Epic 1: Foundation).

Depends on: Nothing — additive, no behavior change.

Context

Epic 4 needs the PARALLEL_TOOL_EXECUTION flag to exist before any behavior change ships. Registering it now is safe and fully additive — it defaults to false and has no effect until Epic 4 checks it. This also allows QA to enable it in staging before the full parallel implementation lands.

Developer Notes

In packages/types/src/experiment.ts:

• Add "parallelToolExecution" to the experimentIds array. The experimentIdsSchema is derived automatically via z.enum(experimentIds), so no separate schema change is needed.

In src/shared/experiments.ts:

• Add PARALLEL_TOOL_EXECUTION: "parallelToolExecution" to EXPERIMENT_IDS.
• Add PARALLEL_TOOL_EXECUTION: { enabled: false } to experimentConfigsMap.

Files: packages/types/src/experiment.ts, src/shared/experiments.ts

Tests (add to src/shared/__tests__/experiments.spec.ts or equivalent):

• experiments.isEnabled({}, "parallelToolExecution") returns false.
• experiments.isEnabled({ parallelToolExecution: true }, "parallelToolExecution") returns true.

Acceptance Criteria

• Default value is false.
• The AssertEqual<Equals<...>> compile-time type check in experiment.ts still passes — this enforces that ExperimentId stays in sync with the Experiments type.
• No existing tests break.