fix: make goal creation idempotent

[danyel117]

Summary

• make repeated create_goal and set_goal calls return the existing matching goal without rewriting state
• return a terminal conflict result for a different non-closed objective instead of the retry-triggering tool error
• allow matching objectives to start fresh after the prior goal is complete or unmet
• preserve Plan/restricted and safety-limit notices on reused goals
• instruct /goal to inspect state and call create_goal at most once
• handle concurrent matching creation calls without duplicate goal history

Validation

• bun run lint
• bun run typecheck
• bun test (208 pass)
• bun run build
• bun run pack:dry-run
• two Claude Opus review rounds; final result: no merge blocker

Closes #21