fix: close the six defects an independent review found in the trial fixes

Independent read-only review of c56f852..2d29966 (2026-10-08), each defect
reproduced by the reviewer; each new test fails on that code.

- test-lock green ran ANY allowlisted command: `green --cmd "git status"`
  marked a red step green and the stop guard let it stop. The lock now
  stores the step's test command (`lock --cmd`); `green` runs only that
  one (a different --cmd, or a lock without one, exits 2). trekexecute D1
  calls `green` with no command, which also removes the quoting hazard.
- plan-gate --accept-open worked at round 1 and passed the gate; it now
  refuses while rounds are left.
- findings-schema --repo-root swallowed a following flag, or took a root
  that does not exist, and silently skipped the line check: exit 2.
- brief-review-stamp hashed only the body, so a changed `framing` or
  `research_topics` stayed "current"; it now covers the reviewed
  frontmatter too (only the post-review stamps are left out).
- red-proof red_kind read an assertion message naming SyntaxError as a
  load error; the signatures are now specific.

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

agents/step-implementer.md

@@ -48,7 +48,9 @@ and never touch a file the step does not list.
    skip steps 1–3, write "no Test first: no test written, no proof of red" in
    the report, and go to step 4.
 3. **Lock the tests** (when the CLI exists):
-   `node {voyage_root}/lib/util/test-lock.mjs lock --files <test files> --step {step}`.
+   `node {voyage_root}/lib/util/test-lock.mjs lock --files <test files> --step {step} --cmd "{Verify command}"`
+   (the lock stores the step's test command; only that command can later
+   mark the step green).
    If the file does not exist, write "not-locked (test-lock.mjs missing)" in the
    report. Never silent. Once locked you cannot edit the tests, and that is the
    point: the code changes to meet the test, never the reverse.