fix(ultraplan-local): v2.3.2 — skill-drafter slug-collision hint

skill-drafter now reads {catalog_root}/<slug>.md before writing its
draft and prepends a warning block to its confirmation output when
an existing skill would be overwritten during manual `mv` promotion.
The draft is still written to .drafts/<slug>.md — the check is a
hint, not a block.

Closes v2.3.0 dogfood finding (post_dogfood_findings[0]): the
drafter produced .drafts/hooks-pattern.md when an approved
hooks-pattern.md seed already existed, giving no signal that `mv`
during promotion would silently overwrite the seed. v2.3.1
introduced the qualified-slug mechanism to resolve such collisions;
v2.3.2 surfaces them at the right moment — before promotion.

Changes:
- agents/skill-drafter.md — new Step 2 between slug computation and
  source reading. Reads {catalog_root}/<slug>.md, inspects
  review_status, derives a kebab-case qualifier from the concept
  handle (or source basename fallback). Subsequent steps renumbered
  3→7. Output format gains Collision: field and optional warning
  block. New Hard Rule.
- tests/fixtures/skill-drafter/slug-collision-expected.md — reference
  fixture documenting expected confirmation shape across four
  scenarios (no collision, approved collision, soft pending
  collision, collision with no good qualifier). Skill-drafter is
  prompt-driven; fixture anchors shape for human verification and
  downstream parsers.
- CHANGELOG [2.3.2], plugin.json 2.3.1→2.3.2, README badge, plugin
  CLAUDE.md slug-convention Collision-hint bullet, marketplace root
  README summary, marketplace root CLAUDE.md plugin table.

Non-breaking. No frontmatter/drafts-layout/tool-scope/regex changes.
Existing pipelines see one extra field and an optional warning —
both purely additive.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/ultraplan-local/CHANGELOG.md

@@ -4,6 +4,50 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
+## [2.3.2] - 2026-04-18
+
+### Fixed — skill-drafter slug-collision hint
+
+`skill-drafter` now checks for an existing file at
+`{catalog_root}/<slug>.md` before writing its draft to `.drafts/`.
+When a collision is detected, the agent prepends a warning block to
+its confirmation output showing the overwrite risk and a suggested
+qualified slug derived from the concept handle. The draft is still
+written to `.drafts/<slug>.md` — the check is a hint, not a block.
+
+**Why.** v2.3.0 dogfood surfaced the risk (logged as
+`post_dogfood_findings[0]` in that run's `progress.json`): when the
+drafter produced `.drafts/hooks-pattern.md` with an existing approved
+`hooks-pattern.md` seed present at the catalog root, the pipeline
+gave no signal that manual `mv` during promotion would silently
+overwrite the seed. The v2.3.1 qualified-slug convention gave us the
+mechanism to resolve collisions, but `skill-drafter` still didn't
+surface them at the right moment — before promotion, not after.
+
+**Changes.**
+
+- `agents/skill-drafter.md` — new Step 2 "Check for slug collision at
+  the catalog root" between slug computation (Step 1) and reading the
+  source (Step 3). Subsequent workflow steps renumbered 3→7. New
+  "Suggesting a qualifier" guidance derives a kebab-case qualifier
+  from the `concept` field (or source basename as fallback). Output
+  format gains a `Collision:` field (`none | approved | pending |
+  auto-merged | soft`) and an optional warning block when the
+  collision is non-none. New Hard Rule "Slug-collision pre-write
+  check".
+- `tests/fixtures/skill-drafter/slug-collision-expected.md` — new
+  reference fixture documenting the expected confirmation-output
+  shape across four scenarios (no collision, approved collision,
+  soft pending collision, collision with no good qualifier).
+  Skill-drafter is prompt-driven and not auto-tested; the fixture
+  anchors the shape for human verification and downstream parsers.
+
+**Non-breaking.** No changes to `.drafts/` layout, frontmatter
+contract, tool scope, or filename regex. Existing pipelines see an
+extra field (`Collision:`) and an optional warning block — both
+purely additive. No version-gated changes in
+`skill-author-orchestrator` or `ip-hygiene-checker`.
+
 ## [2.3.1] - 2026-04-18
 
 ### Added — Qualified slug convention for cc-architect-catalog