fix(pilot): persist AI-proposal rejection + clear on outcome write

Issue #3 from phase-8-review-issues.md. 'Not yet' on the AI-confirming
banner was a local-state hide; the proposal re-surfaced on the next
refreshSessionDerived call.

Two-part fix:
- PATCH /outcome now clears ai_outcome_proposal on any terminal action
  (engineer has taken a decision; stale AI proposal is moot).
- New DELETE /ai-sessions/:sid/suggested-fixes/:fid/ai-outcome-proposal
  endpoint for explicit 'Not yet' rejection. Does not touch status
  or state_version — pure UI state.

Frontend handleRejectAIProposal now calls the DELETE and setActiveFix
with the server response.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/endpoints/session_suggested_fixes.py

@@ -338,6 +338,9 @@ async def patch_suggested_fix_outcome(
     if fix.applied_at is None and body.outcome != "dismissed":
         fix.applied_at = now
 
+    # Clear any pending AI outcome proposal — engineer has taken a terminal action.
+    fix.ai_outcome_proposal = None
+
     # Outcome changes the bundle that resolution-note/escalation-package
     # previews see, so bump state_version inside the same transaction —
     # mirrors the pattern in record_decision above.
@@ -352,6 +355,48 @@ async def patch_suggested_fix_outcome(
     return SessionSuggestedFixResponse.model_validate(fix)
 
 
+# ── Suggested fix: clear AI outcome proposal ("Not yet") ─────────────────────
+
+@router.delete(
+    "/suggested-fixes/{fix_id}/ai-outcome-proposal",
+    response_model=SessionSuggestedFixResponse,
+)
+async def clear_ai_outcome_proposal(
+    session_id: UUID,
+    fix_id: UUID,
+    current_user: Annotated[User, Depends(get_current_active_user)],
+    db: Annotated[AsyncSession, Depends(get_db)],
+    _: None = Depends(require_engineer_or_admin),
+) -> SessionSuggestedFixResponse:
+    """Explicitly dismiss the AI-proposed outcome banner ("Not yet").
+
+    Clears `ai_outcome_proposal` without touching status or state_version
+    (this is pure UI state, not outcome data). Idempotent: returns 200 even
+    when the field is already null. After this call the banner will not
+    re-surface on the next refreshSessionDerived unless the AI emits a new
+    proposal.
+    """
+    await _load_session_or_404(db, session_id)
+
+    result = await db.execute(
+        select(SessionSuggestedFix).where(
+            SessionSuggestedFix.id == fix_id,
+            SessionSuggestedFix.session_id == session_id,
+        )
+    )
+    fix = result.scalar_one_or_none()
+    if fix is None:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND, detail="Suggested fix not found"
+        )
+
+    fix.ai_outcome_proposal = None
+
+    await db.commit()
+    await db.refresh(fix)
+    return SessionSuggestedFixResponse.model_validate(fix)
+
+
 async def _summarize_session_for_extraction(
     db: AsyncSession, session_id: UUID,
 ) -> str: