feat: gate decision-type validation, force-advance, and frontend fixes

Gate (tht-gate.js):
- Pre-validate decision types against workflow.yaml before showing reviewer widget
- Reject decisions emitted by later phases (min-phase check)
- Copy top-level `kind` into artifact when model forgets it (prevents loop)
- Force-advance on reviewer_decide/schema_linking when advance:true — skip
  redundant reviewer_confirm gate

Backend:
- Emit agent_end on clean Pi exit (code 0 + bridge idle) instead of marking failed

Frontend:
- Strip <think> tags from transcript and activity panel
- Fix mermaid render with offscreen container + cleanup
- Graceful mermaid error: show source code instead of red error, fall back to table

Workflow:
- F2 now emits table_promoted and table_excluded (early schema linking decisions)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

harness/tests/test_decision_min_phase.py

@@ -16,7 +16,7 @@ from tht.workflow import load_workflow
         ("concept_clarified", 1),
         ("memory_rejected", 2),
         ("question_rewritten", 3),
-        ("table_promoted", 4),
+        ("table_promoted", 2),
         ("column_corrected", 4),
         ("evidence_accepted", 4),
         ("value_grounded", 4),