feat(webhook): start pipeline on In Progress status (not on create)

Feature 1. work_item.created no longer starts the pipeline (soft QG-0 log only);
the issue stays in the backlog until moved to In Progress. The pipeline-start body
is extracted into start_pipeline(); a new issue updated handler routes a state
change to In Progress -> handle_status_start, which is idempotent: an existing task
for the plane_id is NOT re-created or restarted (protects handle_comment, which also
flips issues to In Progress). Real Plane payload: event=issue, action=updated,
data.state.id. Existing m6/plane_webhook/dedup tests updated to drive the new
trigger; new test_status_trigger.py covers created-no-op / start / idempotent.

tests/test_m6_sequence.py

@@ -102,16 +102,22 @@ def test_fetch_sequence_id_missing_field_returns_none():
 # handle_work_item_created: seq available -> prefix-NNN
 # ---------------------------------------------------------------------------
 
+# Feature 1: pipeline starts on a status change to In Progress, not on creation.
+_IN_PROGRESS = "b873d9eb-993c-48cd-97ac-99a9b1623967"
+
+
 def _post(plane_id, plane_project_id=ORCH_PLANE_ID, name="A valid work item title"):
     return client.post(
         "/webhook/plane",
         json={
-            "event": "work_item.created",
+            "event": "issue",
+            "action": "updated",
             "data": {
                 "id": plane_id,
                 "name": name,
                 "description_stripped": "This is a sufficiently long description.",
                 "project": plane_project_id,
+                "state": {"id": _IN_PROGRESS, "name": "In Progress", "group": "started"},
             },
         },
     )