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>

frontend/src/api/sessionSuggestedFixes.ts

@@ -196,6 +196,18 @@ export const sessionSuggestedFixesApi = {
     )
     return r.data
   },
+
+  /**
+   * Explicitly dismiss the AI-proposed outcome banner ("Not yet").
+   * Clears ai_outcome_proposal on the server without touching status or
+   * state_version. Idempotent: returns 200 even when the field is already null.
+   */
+  async clearAIProposal(sessionId: string, fixId: string): Promise<SessionSuggestedFix> {
+    const r = await apiClient.delete<SessionSuggestedFix>(
+      `/ai-sessions/${sessionId}/suggested-fixes/${fixId}/ai-outcome-proposal`,
+    )
+    return r.data
+  },
 }
 
 export default sessionSuggestedFixesApi