feat(ai_review): parse .pr-review.json:static_message + render as banner

Repos can pin a free-text notice (e.g. 'this repo is in maintenance mode',
or 'reviewers: focus on the public API only for this quarter') in
.pr-review.json:static_message. The string is stripped and capped at 400
chars (mirror of the existing instructions cap), then rendered as a
Markdown blockquote (> {msg}) directly under the REVIEW_HEADER so it
surfaces on every review without scrolling.

Plumbing: parse_repo_config exposes 'static_message'; format_review_body
accepts a static_message kwarg and inserts the blockquote before the
'### Summary of Changes' section; review_pr threads
config.get('static_message') to both call sites (salvage path + happy
path). Empty / non-string values are silently dropped, mirroring the
parser's 'ignore blank' handling for every other text field.

tests/pilot/test_ai_review.py

@@ -401,6 +401,24 @@ def test_parse_repo_config_partial_and_bad():
     assert parse_repo_config('{"instructions":"   "}') == {}
 
 
+def test_parse_repo_config_reads_static_message():
+    cfg = parse_repo_config(json.dumps({"static_message": "  NOTE: this repo is in maintenance mode  "}))
+    assert cfg == {"static_message": "NOTE: this repo is in maintenance mode"}
+
+
+def test_parse_repo_config_static_message_caps_length():
+    long_text = "x" * 9999
+    cfg = parse_repo_config(json.dumps({"static_message": long_text}))
+    assert "static_message" in cfg
+    assert len(cfg["static_message"]) <= 400
+
+
+def test_parse_repo_config_static_message_ignores_blank():
+    assert "static_message" not in parse_repo_config(json.dumps({"static_message": "   "}))
+    assert "static_message" not in parse_repo_config(json.dumps({"static_message": ""}))
+    assert "static_message" not in parse_repo_config(json.dumps({"static_message": 42}))
+
+
 # ---------------------------------------------------------------------------
 # dedupe / prior-context parsing
 # ---------------------------------------------------------------------------
@@ -823,6 +841,22 @@ def test_format_review_body_no_usage_section_omitted():
     assert "AI usage" not in body
 
 
+def test_format_review_body_renders_static_message_banner():
+    body = format_review_body(
+        "", "glm-5.2:cloud", "abcdef1234567890",
+        static_message="NOTE: this repo is in maintenance mode.",
+    )
+    assert "> NOTE: this repo is in maintenance mode." in body
+    # Banner sits under the header and above the rest of the body.
+    assert body.index("NOTE") > body.index("🤖")
+    assert body.index("NOTE") < body.index("### Summary of Changes")
+
+
+def test_format_review_body_omits_static_message_when_blank():
+    body = format_review_body("", "glm-5.2:cloud", "abcdef1234567890")
+    assert "> " not in body
+
+
 def test_format_review_body_with_summary_changes_and_risks():
     body = format_review_body(
         "", "glm-5.2:cloud", "abcdef1234567890",