goal: agent-registered thread goals with harness enforcement (#497)

Replaces the /grind text directive with a first-class goal system: create_goal, get_goal, and update_goal tools register one active goal per thread — only on explicit user request, never inferred — and while a goal is active the harness answers a stop with a continuation prompt carrying a completion-audit discipline (objective wrapped as user data rather than higher-priority instructions, completion treated as unproven, requirement-by-requirement verification, explicit anti-scope-shrink language). Opting out has deliberate friction: 'blocked' requires a stated impasse and is accepted only on the third claim across distinct continuation rounds, 'complete' is refused while the work floor is unmet, a token cap triggers a single wind-down prompt that never fakes completion, and five stalled rounds with zero new tool calls auto-waive with a visible note. Goals persist as session entries and rehydrate on the next turn; the old directive parsing — where a mid-sentence '/grind for 10 minutes' silently did nothing — is removed.

This PR intentionally carries two commits: the first adds the interim /grind directive, the second replaces it with the goal tools. They land together so the directive is superseded within the same PR and never ships in a release on its own.

**Deployment notes**

Goals persist as session system entries and rehydrate on later turns — additive persisted format; old code replaying a session with goal entries ignores them (verify the system-entry replay path tolerates unknown kinds). Rollback: active goals silently stop being enforced, nothing breaks. New harness enforcement loop (turn cannot end while a goal is active) only triggers on explicit create_goal calls — no ambient behavior change.

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