feat(pilot): [FIX_OUTCOME] system prompt instructions

Tells the AI when + how to emit the [FIX_OUTCOME] marker that Task 4's
parser consumes. Placeholder-only per the anti-parrot pattern — no
literal UUIDs, outcomes, or reasons that could leak into unrelated
sessions.

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

backend/tests/test_prompt_anti_parrot.py

@@ -87,7 +87,7 @@ _FORBIDDEN_LITERAL_TOKENS: tuple[str, ...] = (
 #   so prose blocks (like the closing-tag-distance regex match across
 #   markdown headings) are excluded
 _MARKER_BLOCK_RE = re.compile(
-    r"(?:^|\n)\[(QUESTIONS|ACTIONS|SUGGEST_FIX|PROMOTE|FORK|TREE_UPDATE|STEPS_UPDATE|INTAKE_FORM|METADATA|DELTA)\]"
+    r"(?:^|\n)\[(QUESTIONS|ACTIONS|SUGGEST_FIX|FIX_OUTCOME|PROMOTE|FORK|TREE_UPDATE|STEPS_UPDATE|INTAKE_FORM|METADATA|DELTA)\]"
     r"\s*\n"                          # forced newline before content
     r"(\s*[\[{][\s\S]*?)"             # content must start with [ or {
     r"\s*\n\[/\1\]"