feat(#1318): require external reviewers to verify plan claims against source (#1421)

* feat(#1318): require external reviewers to verify plan claims against source

/gsd-review built its external-reviewer prompt from plan text only and never
asked reviewers to open the repo and verify claims, so a grounded HIGH could be
outvoted by ungrounded LOWs. Add a concise, generic source-grounding block to
build_prompt's Review Instructions: treat yourself as running in the working
tree, open referenced files, cite path:line + mechanism, trace asserted
mechanisms, downgrade to an open question if you have no file access, and know
that grounded findings are weighted more heavily.

Also clarify that CodeRabbit (a diff-only reviewer that never receives the
prompt) must not be weighted as a grounded plan-level verdict in consensus
synthesis. Workflow stays under its size cap (baseline bumped deliberately).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* chore(#1318): add changeset for reviewer source-grounding

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* chore(#1318): mark changeset docs-exempt (internal reviewer-prompt wording)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs(#1318): document reviewer source-grounding in COMMANDS.md; drop docs-exempt

Review: a user-visible behavioral Changed warrants a docs touch, not a
docs-exempt. Add a sentence to the /gsd-review entry in docs/COMMANDS.md
(reviewers verify against source, cite file:line, grounded findings weighted
higher) and remove the changeset docs-exempt marker so lint:docs passes via
docs-updated. Also note the literal build_prompt test anchor.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* test(#1318): harden build_prompt fence extraction to be fence-run-aware

Addresses maintainer review on PR #1421 (required-before-merge).

The buildPromptReviewInstructions() test helper located the closing fence with
`src.indexOf('\n```')`, which terminates at the FIRST triple-backtick line — so
a build_prompt ```markdown block whose body embeds a fenced code example would
truncate mid-content (dropping the `## Review Instructions` section) and give a
spurious failure or false pass. Since this feature feeds source/plan content
(which routinely contains code fences) to reviewers, that is a live fragility.

Rewrite the extraction to be fence-run-aware, mirroring the CommonMark close
rule in src/markdown-sectionizer.cts stripFencedCode: parse the opener's
backtick run length, then close on the first line with >= that many backticks
and only trailing whitespace — so a shorter nested fence is treated as content.
Add a fail-first regression test (a 4-backtick outer fence wrapping a nested
```bash block) asserting the trailing `## Review Instructions` still extracts.

Test-only change; no production .cts touched. Verified: test file 7/7,
empirical fail-first proof the old indexOf logic truncated, full suite
4236/4236, eslint clean. Codex review: approve.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Tom Boucher <trekkie@nomorestars.com>

docs/COMMANDS.md

@@ -1370,6 +1370,8 @@ Execute a trivial task inline — no subagents, no planning overhead. For typo f
 
 Cross-AI peer review of phase plans from external AI CLIs.
 
+Reviewers are prompted to verify the plan's claims against the actual repository source — opening the referenced files and citing `file:line` evidence with the mechanism — rather than reviewing the plan text in isolation. A reviewer that has no file access flags what it cannot verify instead of asserting it, and `file:line`-grounded findings are weighted more heavily during consensus synthesis.
+
 | Argument | Required | Description |
 |----------|----------|-------------|
 | `--phase N` | **Yes** | Phase number to review |