fix(#2494): capture stderr and guard empty output on the claude and gemini reviewer legs (#2592)

* fix(#2494): capture stderr and guard empty output on the claude and gemini reviewer legs

The gemini and claude blocks in gsd-core/workflows/review.md were the only
two of the ten prompt-fed reviewer legs with both `2>/dev/null` and no
empty-output guard. Any failure that wrote no stdout — CLI missing,
unauthenticated, rate-limited, crashed — left a zero-byte review file with
the only diagnostic evidence already discarded.

write_reviews substitutes each file's raw content into a `## <Reviewer>
Review` section with no marker separating "empty/failed" from "ran cleanly,
nothing to report", so a failed lane silently degraded the advertised
N-reviewer consensus to N-1 while present_results reported success. Both
/gsd:review and /gsd:plan-review-convergence share the invoke_reviewers
step, so both were affected.

Both legs now redirect stderr to a `.err` sidecar and write a diagnostic
stub with the captured stderr appended when the review file comes back
empty — the same shape the codex and cursor legs already use.

Scope limit, noted in the block comment: the guard only runs if the block
itself completes. A host Bash-tool timeout that kills the whole block skips
it, the same hard bound the OpenCode block already documents. The existing
timeout guidance (#2194) covers that case and is unchanged.

Regression test extracts the two dispatch blocks verbatim from the workflow
and runs them under bash against a failing CLI stub, asserting the review
file is non-empty and carries a diagnosable message plus the captured
stderr. It fails against pre-fix review.md (4 of 5 cases).

Golden install-parity fixtures and the workflow size baseline are
regenerated: one review.md hash per fixture, one size entry.

Fixes #2494

* fix(#2494): stamp the changeset fragment with the filed PR number

The fragment was authored with the documented `pr: 0` placeholder because
the PR number does not exist until `gh pr create` returns. Now that this
PR is #2592, stamp it — scripts/changeset/parse.cjs requires pr > 0, so
the placeholder would fail the Changeset Required check.

tests/workflow-size-baseline.json

@@ -64,7 +64,7 @@
   "remove-phase.md": 8513,
   "remove-workspace.md": 7916,
   "resume-project.md": 17270,
-  "review.md": 51210,
+  "review.md": 52646,
   "scan.md": 8314,
   "secure-phase.md": 14627,
   "session-report.md": 4044,