fix(consistency): fix C-04 status vocabulary mismatch + surface PATCH errors

Root cause: workplan files use "done" (task vocabulary) but the DB workstream
API only accepts "completed". The PATCH was silently failing with 422.

Fixes:
- Add FILE_TO_DB_WORKSTREAM_STATUS map and normalise_workstream_status()
- Normalise file status before C-04 comparison: done↔completed is no longer
  spurious drift
- Normalise file status before PATCHing: always send DB-valid "completed"
- _api_patch now returns {"_error": ...} instead of None on failure, so the
  fix loop reports FAILED entries rather than silently dropping them
- 9 new tests in TestNormaliseWorkstreamStatus (42 total)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_consistency_check.py

@@ -23,7 +23,9 @@ sys.path.insert(0, str(Path(__file__).parent.parent / "scripts"))
 from consistency_check import (
     ConsistencyReport,
     Issue,
+    FILE_TO_DB_WORKSTREAM_STATUS,
     get_tasks_from_workplan,
+    normalise_workstream_status,
     parse_frontmatter,
     parse_task_blocks,
     render_text,
@@ -326,3 +328,46 @@ class TestReportToDict:
         d = report_to_dict(r)
         assert d["repo_slug"] == "the-custodian"
         assert d["repo_path"] == "/home/worsch/the-custodian"
+
+
+# ---------------------------------------------------------------------------
+# Status vocabulary normalisation
+# ---------------------------------------------------------------------------
+
+class TestNormaliseWorkstreamStatus:
+    """FILE_TO_DB_WORKSTREAM_STATUS maps workplan file vocabulary to DB vocabulary.
+
+    Workplan files use task-style "done"; the DB workstream API uses "completed".
+    The C-04 check and fix code must normalise before comparing or PATCHing.
+    """
+
+    def test_done_maps_to_completed(self):
+        assert normalise_workstream_status("done") == "completed"
+
+    def test_completed_is_identity(self):
+        assert normalise_workstream_status("completed") == "completed"
+
+    def test_active_is_identity(self):
+        assert normalise_workstream_status("active") == "active"
+
+    def test_blocked_is_identity(self):
+        assert normalise_workstream_status("blocked") == "blocked"
+
+    def test_archived_is_identity(self):
+        assert normalise_workstream_status("archived") == "archived"
+
+    def test_unknown_value_returned_as_is(self):
+        # Don't crash on unexpected values — return them unchanged
+        assert normalise_workstream_status("foobar") == "foobar"
+
+    def test_map_constant_covers_done(self):
+        assert "done" in FILE_TO_DB_WORKSTREAM_STATUS
+        assert FILE_TO_DB_WORKSTREAM_STATUS["done"] == "completed"
+
+    def test_c04_no_spurious_drift_when_done_vs_completed(self):
+        """done (file) vs completed (DB) must NOT be reported as C-04 drift."""
+        assert normalise_workstream_status("done") == normalise_workstream_status("completed")
+
+    def test_c04_real_drift_still_detected(self):
+        """done (file) vs active (DB) IS real drift and must be detected."""
+        assert normalise_workstream_status("done") != normalise_workstream_status("active")