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/workflow.yaml

@@ -21,7 +21,7 @@ phases:
     advance: auto_if_empty
     prerequisites: []
     artifacts_out: []
-    emits: [memory_rejected]
+    emits: [memory_rejected, table_promoted, table_excluded]
   - id: F3
     name: riscrittura
     advance: kind:phase