fix(#571): forbid Write in doc-writer fix mode; add workflow truncation guard

* fix(#571): forbid Write in doc-writer fix mode; add workflow truncation guard

gsd-doc-writer in fix mode only had Write in its tools list, so when
correcting a specific failing claim it would re-emit the whole file with
only the lines it had in context — truncating untracked docs with no git
recovery path.

Fix 1 (root cause): add Edit to the agent tools frontmatter and rewrite
fix_mode instructions to mandate Edit for surgical corrections and
explicitly forbid Write on existing files. Also reinforced in
critical_rules.

Fix 2 (safety net): add a post-fix line-count guard in the fix_loop step
of docs-update.md. If the file shrank by >90% after a fix agent runs,
the orchestrator restores the file from the existing_content it captured
before dispatch and logs a WARNING. This makes the previously
unrecoverable case recoverable.

Regression test: tests/bug-571-doc-writer-fix-mode-edit-only.test.cjs
covers both the agent contract and the workflow guard.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore: add changeset for fix #571

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(#571): address codex adversarial review findings

- Quote {doc_path} in shell snippets to handle paths with spaces (#SECURITY)
- Clarify corrupted doc re-verification vs re-fix distinction (#CORRECTNESS)
- Strengthen regression tests with structural ordering assertions (#REGRESSION)
- Move docs-update.md from global ALLOWLIST to SIZE_ONLY_WORKFLOWS so
  injection scanning still runs while only the 50K size finding is exempt (#SECURITY)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.changeset/sturdy-pumas-sing.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 571
+---
+gsd-doc-writer fix mode now uses the Edit tool for surgical corrections instead of Write. Previously, the fix_loop in docs-update.md could call gsd-doc-writer in fix mode with Write, truncating an untracked doc to a single line with no git recovery path. Adds a post-fix line-count guard in fix_loop that restores from pre-fix content if >90% shrinkage is detected.