feat: parallel milestone orchestration foundation (#672)
Add infrastructure for parallel milestone execution behind
`parallel.enabled: false` flag (opt-in, zero impact to existing users).
New modules:
- session-status-io.ts: File-based IPC protocol with atomic writes,
signal lifecycle (pause/resume/stop), and stale session detection
- parallel-eligibility.ts: Milestone parallelism analysis checking
dependency satisfaction and file overlap across slice plans
- parallel-orchestrator.ts: Core orchestrator managing worker lifecycle,
budget tracking, and coordination via session status files
- /gsd parallel [start|status|stop|pause|resume] command handlers
Modified:
- types.ts: ParallelConfig interface (enabled, max_workers, budget_ceiling,
merge_strategy, auto_merge)
- preferences.ts: Parallel config validation, merging, and resolver
- commands.ts: /gsd parallel subcommand routing with argument completions
Tests: 39 new tests covering session I/O roundtrip, signal lifecycle,
stale detection, eligibility formatting, orchestrator lifecycle,
budget enforcement, and preference validation.
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