fix(work-item): prevent work_item_id collision and bind branch per task

ET-006 was handed to two different tasks because M-6 derives work_item_id from
the Plane sequence_id, which can collide -> the two tasks shared a branch/worktree
slug prefix and stepped on each other.

2a: ensure_unique_work_item_id() is a uniqueness-guard LAYERED ON TOP of the M-6
derive (derive is untouched): if the derived ET-NNN already exists in tasks for
the repo, it walks forward to the next free number. Applied in start_pipeline
after the derive.

2b (defense-in-depth): worktree is keyed by branch; if the resulting branch is
already owned by another task in the repo, disambiguate it with the unique
work_item_id + plane id so two tasks can never share a worktree.

src/db.py

@@ -159,6 +159,44 @@ def get_next_work_item_id(repo: str, prefix: str = "ET") -> str:
     return f"{prefix}-{next_num:03d}"
 
 
+def ensure_unique_work_item_id(work_item_id: str, repo: str) -> str:
+    """BUG 2a: guarantee work_item_id uniqueness within (repo) over M-6 derive.
+
+    M-6 derives the work_item_id from the Plane sequence_id. That number can
+    collide (e.g. an issue was deleted and the sequence reused, or two issues
+    map to the same number) -> the SAME ET-NNN gets handed to two different
+    tasks, which then physically share a branch/worktree slug prefix and step on
+    each other (see ET-006: task 8 and task 25).
+
+    This is a guard LAYERED ON TOP of the M-6 derive (it does NOT replace it):
+    given the derived id, if that exact <PREFIX>-NNN already exists in the tasks
+    table for this repo, walk forward (ET-007, ET-008, ...) until a free number
+    is found and return that instead. If the derived id is free, it is returned
+    unchanged.
+    """
+    if not work_item_id or "-" not in work_item_id:
+        return work_item_id
+    prefix, num_str = work_item_id.rsplit("-", 1)
+    try:
+        num = int(num_str)
+    except ValueError:
+        return work_item_id
+    width = len(num_str)
+
+    conn = get_db()
+    try:
+        candidate = work_item_id
+        while conn.execute(
+            "SELECT 1 FROM tasks WHERE repo = ? AND work_item_id = ? LIMIT 1",
+            (repo, candidate),
+        ).fetchone() is not None:
+            num += 1
+            candidate = f"{prefix}-{num:0{width}d}"
+        return candidate
+    finally:
+        conn.close()
+
+
 # ---------------------------------------------------------------------------
 # ORCH-5 (M-7): idempotent webhook event logging
 # ---------------------------------------------------------------------------