feat(trekplan): Regression and On failure step rules, and trial 2's minor fixes

The minor points (B8-B12, C13, C14, C16, C17) of the second voyage-lekerepo
trial (2026-10-08), each verified against the code and started with a test
that failed first.

- K10: optional step field `Regression:` - one command the criteria runner
  allows, unpiped; plan-validator refuses one that can never run
  (PLAN_STEP_REGRESSION_NOT_RUNNABLE). Any plan_version, no bump.
- K11: `On failure:` opens with revert | retry | skip | escalate, or the
  validator warns (PLAN_STEP_ON_FAILURE_UNKNOWN); onFailureAction in
  plan-schema.
- B8 (operator decision 2026-10-08): minors open after the last critic round
  are listed numbered in the Phase 10 summary; the ones the operator picks are
  fixed and get one more round (--extra-round when the 3 are spent).
- B11: plan-review dedup merges two reviewers' findings on the same file:line
  with the same content (trial pair: Jaccard 0.44, overlap 0.83); the Jaccard
  threshold for every other pair is unchanged.
- B12: /trekbrief Step 4c says the reviewer runs in the background and is
  waited for, not "foreground, blocking".
- C13: the Phase 4 size count includes .mjs, .cjs, .mts, .cts.
- C14: --evidence shows the first output line on PASS rows.
- C16: findings-schema checks path:line citations in finding prose (error) and
  in the prose around the block (warning) against the file.
- C17: git-historian gets the brief, never proposes changing a frozen file,
  and names the remote from `git remote -v`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/parsers/plan-schema.mjs

@@ -263,6 +263,24 @@ export function parseStepFields(body) {
   return fields;
 }
 
+// ---- `On failure:` (K11) ---------------------------------------------------
+
+// The recovery words /trekexecute reads from a step's `On failure:` ("Parse
+// steps"). Free text that opens with none of them left the executor to guess:
+// "retry once; if still failing, revert ... and STOP" met a ladder of fix rounds
+// (voyage-lekerepo trial 2, 2026-10-08).
+export const ON_FAILURE_ACTIONS = Object.freeze(['revert', 'retry', 'skip', 'escalate']);
+
+/**
+ * The recovery word an `On failure:` text opens with, or null. Case-insensitive;
+ * leading markdown (`**`, backticks, a sub-bullet's `-`) is stripped.
+ */
+export function onFailureAction(text) {
+  const s = String(text ?? '').trim().replace(/^[*_`\s-]+/, '').toLowerCase();
+  const m = s.match(/^([a-z]+)\b/);
+  return m && ON_FAILURE_ACTIONS.includes(m[1]) ? m[1] : null;
+}
+
 // ---- sections (fence-aware) ------------------------------------------------
 
 /**