loops: reorder rationale extraction — REORDER: markers logged per iteration

Coordinator can now log WHY it worked on an INT-XX out of order
("REORDER: INT-05 before INT-04 because prereq X is unmet") and the
completion hook captures each rationale as an append-only event on
the loop. Sets up a reviewable timeline of when the plan was
adjusted, independent of the underlying `consumed_int_ids` advance.

Migration 0043:
- loops.reorder_events JSONB NOT NULL DEFAULT '[]'::jsonb — append-
  only array of {run_id, iteration, text, ts}. Kept on the loop row
  (rather than a dedicated table) so the mini-timeline is one read
  away from the loop card.

Backend:
- topology_worker::parse_reorder_rationale — line matcher symmetric
  with parse_completed_int_ids. Tolerates list dashes / prefixes /
  markdown emphasis; case-insensitive marker match, preserves case of
  the rationale text.
- cm_db::repo::loops::append_reorder_event — one INSERT-like append
  per rationale, uses jsonb_build_object with postgres now() so ts is
  wall-clock canonical (no client-clock skew).
- topology_runs::iteration_for_run — new helper so events carry the
  iteration index.
- routes::loops::compose_iteration_task — coordinator prompt now
  explicitly asks for `REORDER: <one-sentence>` at the top of the
  first substantive turn when working out of order, AND spells out
  that both markers must appear literally with colons (no bold, no
  code fence) so the line parser doesn't miss them.

Non-loop and standalone-loop runs are unaffected — the hook only
fires when the run belongs to a source-bound loop.

Follow-up: expose reorder_events on the loops list endpoint + render
a small collapsed timeline on the LoopsList card.

crates/cm-api/src/routes/loops.rs

@@ -79,10 +79,13 @@ async fn compose_iteration_task(pool: &PgPool, loop_id: Uuid, task_template: &st
          - Already completed: {}.\n\
          - Address the NEXT unconsumed INT-XX item in the artifact, in order.\n\
          - If the next item has unmet prerequisites, work on the smallest\n\
-           unblocking INT-XX instead AND log the reorder rationale in your\n\
-           opening turn so the reviewer can trace it.\n\
+           unblocking INT-XX instead. When you reorder, emit a line\n\
+           `REORDER: <one-sentence rationale>` at the top of your first\n\
+           substantive turn — the loop indexes these for a review timeline.\n\
          - Emit `COMPLETED: INT-<NN>` on its own line at the end of the run\n\
-           when the item is done — the loop advances on that marker.\n\n\
+           when the item is done — the loop advances on that marker.\n\
+         - Both markers must appear literally with the colon (no bold, no\n\
+           code fence); the parser is line-based.\n\n\
          ORIGINAL TASK TEMPLATE:\n{}\n",
         outcome.version, outcome.body_md, current_idx, consumed_list, task_template
     )