docs(trekresearch): deep-research engine is operator-only from CC 2.1.218; document the real window

Claude Code 2.1.218 changed /deep-research to start only when invoked
manually; from there the Skill tool refuses a model invocation outright
with `disable-model-invocation` (measured in a real `--engine deep-research`
run 2026-09-01 18:32Z - the SC3 fallback to swarm held). The engine was
built against 2.1.196 and the prose still promised a path Claude Code has
removed ("requires Claude Code 2.1.154+").

The flag is KEPT as an additive opt-in that never hard-fails. Prose only:
no lib/ change, no adapter-contract change, no default change, no version
bump.

- commands/trekresearch.md: flag bullet states the closed window
  `2.1.154 <= CC < 2.1.218`; the pre-gate gained an UPPER ceiling (still a
  numeric comparison, not a string one) that skips straight to the fallback
  with reason `disable-model-invocation`; step 3 names the Skill-tool
  refusal as a known failure manifestation; step 4 lists the reason tokens.
- docs/command-modes.md + README.md: same window, one sentence each, on the
  reference row itself.
- CHANGELOG.md: Unreleased/Docs entry.

Tests first (Iron Law): tests/lib/doc-consistency.test.mjs gained a pin
requiring 2.1.218 + disable-model-invocation on all three surfaces, on both
reference rows, and in both load-bearing regions of the command (flag
bullet + pre-gate). Seen red on all three surfaces before the prose change.
tests/commands/trekresearch-engine.test.mjs unchanged and green.

New baseline: 1024 tests, 1022 pass / 0 fail / 2 skip (was 1023: 1021/0/2).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/lib/doc-consistency.test.mjs

@@ -1117,6 +1117,57 @@ test('deep-research-engine: --engine is documented + consistent across surfaces'
   );
 });
 
+test('deep-research-engine: the CC 2.1.218 operator-only ceiling is documented on every surface', () => {
+  // CC 2.1.218 changed /deep-research to "start only when invoked manually"; from
+  // there the Skill tool refuses a model invocation outright with
+  // `disable-model-invocation`. The flag stays (additive opt-in, always degrades to
+  // swarm), but no surface may keep promising a path Claude Code has removed, and
+  // none may leave `2.1.154+` standing alone as the engine's requirement.
+  const SURFACES = ['commands/trekresearch.md', 'docs/command-modes.md', 'README.md'];
+  for (const f of SURFACES) {
+    const body = read(f);
+    assert.ok(
+      body.includes('2.1.218'),
+      `${f} must document the CC 2.1.218 ceiling for the deep-research engine`,
+    );
+    assert.ok(
+      body.includes('disable-model-invocation'),
+      `${f} must name disable-model-invocation as the fallback reason from CC 2.1.218`,
+    );
+  }
+
+  // The one-line reference rows carry the whole truth on their own line — an
+  // operator reading the flag table never scrolls to the command prose.
+  const ROWS = [
+    ['docs/command-modes.md', /^.*`--engine \{swarm\\\|deep-research\}`.*$/m],
+    ['README.md', /^\|\s*\*\*Engine\*\*.*$/m],
+  ];
+  for (const [f, re] of ROWS) {
+    const m = read(f).match(re);
+    assert.ok(m, `${f} must still carry the --engine reference row`);
+    assert.ok(
+      m[0].includes('2.1.218') && m[0].includes('disable-model-invocation'),
+      `${f} --engine row must state the real window (2.1.218 + disable-model-invocation)`,
+    );
+  }
+
+  // Both load-bearing regions of the command itself: the flag bullet and the
+  // pre-gate that decides whether the delegation is attempted at all.
+  const research = read('commands/trekresearch.md');
+  const bullet = research.match(/8\. `--engine <name>`[\s\S]*?Flags can be combined/);
+  assert.ok(bullet, 'trekresearch.md must still carry the --engine flag bullet');
+  assert.ok(
+    bullet[0].includes('2.1.218') && bullet[0].includes('disable-model-invocation'),
+    'trekresearch.md --engine bullet must state the real window, not just the 2.1.154 floor',
+  );
+  const pregate = research.match(/\*\*Coarse pre-gate[\s\S]*?authoritative guard\./);
+  assert.ok(pregate, 'trekresearch.md must still carry the deep-research pre-gate');
+  assert.ok(
+    pregate[0].includes('2.1.218'),
+    'the pre-gate must carry an upper ceiling at 2.1.218, not only the 2.1.154 floor',
+  );
+});
+
 // ── STORM bounded loop — env-vars documented across the four surfaces ──────
 // Same cross-doc shape as the --engine pin above. An operator-facing switch
 // documented on one surface is a switch most operators never find; and the