feat(hooks): the merge question says how the run was tested and how it is undone

tests/lib/fix-round-4-5b2-b-prose.test.mjs

@@ -32,6 +32,10 @@
 //   DPR  /trekreview Phase 4 says a changed instruction file at any depth,
 //        `docs/**` included, gets deep review, after the triage table and
 //        with its two pinned rows untouched.
+//   MEV  /trekexecute Phase 8 writes the merge evidence file in a Bash call of
+//        its own before the merge call, and goes on when it is not written;
+//        docs/HANDOVER-CONTRACTS.md documents the file as internal: its path
+//        under the git common dir, never tracked, not part of Handover 5.
 // The helpers below are copied, never imported from an earlier prose test.
 // Fix the SOURCE a pin guards, not the pin.
 
@@ -395,3 +399,47 @@ test('[DPR] /trekreview Phase 4 says a changed instruction file at any depth, do
   assert.ok(lines.includes('| `summary-only` | `docs/**`, Markdown files at the repo root other than `CLAUDE.md` and `AGENTS.md` (README, CHANGELOG), `tests/fixtures/**`. |'),
     'Control: the summary-only row is unchanged');
 });
+
+// --- MEV: the merge question's evidence -------------------------------------------
+
+const PHASE_8 = '## Phase 8 — Final report';
+const EVIDENCE_CMD = 'node ${CLAUDE_PLUGIN_ROOT}/lib/review/merge-evidence.mjs write --repo . --branch "{run branch}" --project "{project_dir}" --json';
+
+test('[MEV] Phase 8 writes the evidence file before the merge call', () => {
+  const p8 = section(read(EXEC_CMD), PHASE_8);
+  const open = p8.indexOf('**2. Open: the merge, and the one question.**');
+  assert.ok(open >= 0, 'Phase 8 has its "2. Open" part');
+  const lines = p8.split('\n');
+  const at = lines.findIndex((l) => l.trim() === EVIDENCE_CMD);
+  assert.ok(at >= 0, `Phase 8 runs the evidence writer: ${EVIDENCE_CMD}`);
+  // A Bash call of its own: the command is the one line of its fenced block.
+  assert.match(lines[at - 1].trim(), /^```bash$/, 'the writer opens its own bash block');
+  assert.equal(lines[at + 1].trim(), '```', 'the writer is the only line of its block');
+  const writer = p8.indexOf(EVIDENCE_CMD);
+  assert.ok(writer > open, 'the writer runs in "2. Open"');
+  const merges = [p8.indexOf('merge --no-ff "{run branch}"'), p8.indexOf('git switch "{main}"')];
+  for (const m of merges) assert.ok(m > writer, 'the writer runs before each merge block');
+  assert.ok(sentences(section(read(EXEC_CMD), PHASE_8)).some((s) => s.includes('On `"written": false` (exit 1), print its `reason` in one line and continue')
+    && /no run evidence/.test(s)), 'a refused write is said in one line and the run goes on; the question then says "no run evidence"');
+  // Control: the merge blocks are unchanged.
+  assert.ok(p8.includes('  if git -C "{main worktree path}" merge --no-ff "{run branch}" -m "merge: trekexecute {slug}"; then\n    echo "MERGED"'),
+    'Control: the worktree merge block is unchanged');
+  assert.ok(p8.includes('  git switch "{main}" || { echo "MERGE_REFUSED"; exit 1; }\n  if git merge --no-ff "{run branch}" -m "merge: trekexecute {slug}"; then'),
+    'Control: the switch merge block is unchanged');
+});
+
+test('[MEV] docs/HANDOVER-CONTRACTS.md documents the evidence file as internal', () => {
+  const text = read(CONTRACTS);
+  const note = section(text, '## Internal files');
+  const f = flat(note);
+  assert.ok(f.includes('`<git common dir>/voyage/merge-evidence/<branch head sha>.json`'), 'the note names the path under the git common dir');
+  assert.match(f, /never tracked/, 'never tracked');
+  assert.match(f, /never published/, 'never published');
+  assert.match(f, /not part of Handover 5/, 'not part of Handover 5');
+  for (const field of ['`v`', '`branch`', '`branch_head_sha`', '`written_at`', '`end_to_end`', '`verification`', '`review`', '`door`']) {
+    assert.ok(f.includes(field), `the note names the field ${field}`);
+  }
+  assert.ok(f.includes('lib/review/merge-evidence.mjs') && f.includes('hooks/lib/merge-evidence.mjs'), 'the note names the writer and the reader');
+  // Control: Handover 5 does not name the file.
+  assert.doesNotMatch(section(text, '## Handover 5'), /merge-evidence/, 'Control: Handover 5 is unchanged');
+});