fix(#973): add Edit to gsd-planner tools and forbid whole-file Write of ROADMAP.md (#989)

* fix(#973): add Edit to gsd-planner tools and forbid whole-file Write of ROADMAP.md

gsd-planner shipped Write but not Edit — the same writer-agent gap fixed for six
agents in #571/#581. Without Edit, an in-place ROADMAP update fell back to a
whole-file Write that truncated committed milestone history (292→16 lines in a
real incident).

Changes:
- agents/gsd-planner.md: add Edit to tools: frontmatter (adjacent to Write)
- agents/gsd-planner.md: update_roadmap step now directs Edit (scoped), with an
  explicit blocking prohibition on whole-file Write of ROADMAP.md or any existing
  curated .planning/ file
- agents/gsd-planner.md: Write contract section clarifies Write is authorized only
  for net-new PLAN.md creation; existing files must use Edit
- tests/agent-frontmatter.test.cjs: extend SECTION_WRITER_AGENTS list (#581 test)
  to cover gsd-planner — fails before fix, passes after
- .changeset/973-gsd-planner-edit-tool.md: Fixed changeset, pr:0

Closes #973

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

* chore(#973): backfill changeset pr number (989)

* fix(#973): trim gsd-planner.md prose under agent size cap (keep Edit + scoped-Edit-for-ROADMAP rule)

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

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/AGENTS.md

@@ -161,7 +161,7 @@ GSD uses a multi-agent architecture where thin orchestrators (workflow files) sp
 |----------|-------|
 | **Spawned by** | `/gsd-plan-phase`, `/gsd-quick` |
 | **Parallelism** | Single instance |
-| **Tools** | Read, Write, Bash, Glob, Grep, WebFetch, mcp (context7) |
+| **Tools** | Read, Write, Edit, Bash, Glob, Grep, WebFetch, mcp (context7) |
 | **Model (balanced)** | Opus |
 | **Color** | Green |
 | **Produces** | `{phase}-{N}-PLAN.md` files |