feat(pilot): [FIX_OUTCOME] marker parser + AI outcome proposal

The AI emits [FIX_OUTCOME] when the engineer indicates in chat that a
prior suggested fix worked, didn't work, or was partially applied. The
marker writes to session_suggested_fixes.ai_outcome_proposal (JSONB),
which the frontend surfaces as a "confirm outcome?" banner. The status
column is only updated when the engineer clicks confirm (via PATCH
/outcome endpoint from Task 3).

Placeholder-only system prompt wiring comes in Task 5.

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

backend/tests/test_fix_outcome_marker.py

@@ -0,0 +1,91 @@
+"""Unit tests for the [FIX_OUTCOME] marker parser."""
+from __future__ import annotations
+
+from app.services.unified_chat_service import _parse_fix_outcome_marker
+
+
+def test_parses_success_outcome():
+    ai = (
+        "Great news — that confirms the root cause.\n\n"
+        "[FIX_OUTCOME]\n"
+        '{"fix_id":"11111111-1111-1111-1111-111111111111",'
+        '"outcome":"success","reason":"user said the fix worked"}\n'
+        "[/FIX_OUTCOME]\n"
+    )
+    cleaned, parsed = _parse_fix_outcome_marker(ai)
+    assert "[FIX_OUTCOME]" not in cleaned
+    assert "confirms the root cause" in cleaned
+    assert parsed == {
+        "fix_id": "11111111-1111-1111-1111-111111111111",
+        "outcome": "success",
+        "reason": "user said the fix worked",
+    }
+
+
+def test_parses_failure_outcome():
+    ai = (
+        "[FIX_OUTCOME]\n"
+        '{"fix_id":"22222222-2222-2222-2222-222222222222",'
+        '"outcome":"failure","reason":"user reports still broken"}\n'
+        "[/FIX_OUTCOME]"
+    )
+    cleaned, parsed = _parse_fix_outcome_marker(ai)
+    assert "[FIX_OUTCOME]" not in cleaned
+    assert parsed["outcome"] == "failure"
+
+
+def test_missing_marker_returns_none():
+    ai = "no marker here"
+    cleaned, parsed = _parse_fix_outcome_marker(ai)
+    assert cleaned == ai
+    assert parsed is None
+
+
+def test_invalid_json_is_dropped():
+    ai = "[FIX_OUTCOME]\nnot-json\n[/FIX_OUTCOME]"
+    cleaned, parsed = _parse_fix_outcome_marker(ai)
+    assert "[FIX_OUTCOME]" not in cleaned
+    assert parsed is None
+
+
+def test_unknown_outcome_rejected():
+    ai = (
+        "[FIX_OUTCOME]\n"
+        '{"fix_id":"33333333-3333-3333-3333-333333333333",'
+        '"outcome":"maybe","reason":"x"}\n'
+        "[/FIX_OUTCOME]"
+    )
+    _, parsed = _parse_fix_outcome_marker(ai)
+    assert parsed is None
+
+
+def test_last_block_wins_when_multiple():
+    ai = (
+        "[FIX_OUTCOME]\n"
+        '{"fix_id":"44444444-4444-4444-4444-444444444444",'
+        '"outcome":"failure","reason":"first"}\n'
+        "[/FIX_OUTCOME]\n"
+        "[FIX_OUTCOME]\n"
+        '{"fix_id":"55555555-5555-5555-5555-555555555555",'
+        '"outcome":"success","reason":"second"}\n'
+        "[/FIX_OUTCOME]"
+    )
+    cleaned, parsed = _parse_fix_outcome_marker(ai)
+    assert "[FIX_OUTCOME]" not in cleaned
+    assert parsed["fix_id"] == "55555555-5555-5555-5555-555555555555"
+    assert parsed["outcome"] == "success"
+
+
+def test_parses_partial_outcome():
+    ai = (
+        "[FIX_OUTCOME]\n"
+        '{"fix_id":"66666666-6666-6666-6666-666666666666",'
+        '"outcome":"partial","reason":"user ran cred clear only"}\n'
+        "[/FIX_OUTCOME]"
+    )
+    _, parsed = _parse_fix_outcome_marker(ai)
+    assert parsed == {
+        "fix_id": "66666666-6666-6666-6666-666666666666",
+        "outcome": "partial",
+        "reason": "user ran cred clear only",
+    }