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/README.md

@@ -1,6 +1,6 @@
 # ultraplan-local — Brief, Research, Architect, Plan, Execute
 
-![Version](https://img.shields.io/badge/version-2.3.1-blue)
+![Version](https://img.shields.io/badge/version-2.3.2-blue)
 ![License](https://img.shields.io/badge/license-MIT-green)
 ![Platform](https://img.shields.io/badge/platform-Claude%20Code-purple)
 
@@ -226,7 +226,7 @@ Downstream: `/ultraplan-local` auto-discovers `architecture/overview.md` in proj
 
 The architect command reads from `skills/cc-architect-catalog/`, a plugin-internal catalog of hand-written skills covering each CC feature at one or more layers (`reference`, `pattern`, `decision`). v2.2 shipped 10 seeds covering all 8 features at the reference layer (plus pattern layer for hooks and subagents). The catalog is designed for expansion via the skill-factory process (v2.3) — gaps surfaced by the architect command are the backlog for that work.
 
-**Slug convention (v2.3):** files follow `<cc_feature>[-<qualifier>]-<layer>.md`. Unqualified slugs (e.g., `hooks-pattern.md`) are the baseline/canonical entry for a `(feature, layer)` pair. Qualified slugs (e.g., `hooks-observability-pattern.md`) cover specific sub-patterns without displacing the baseline. `feature-matcher` prefers the baseline when the brief does not specifically justify a qualified variant; it may propose multiple supporting skills when they cover non-overlapping aspects. Slug collisions with approved skills are a hard error — skill-factory drafts must qualify or revise instead.
+**Slug convention (v2.3):** files follow `<cc_feature>[-<qualifier>]-<layer>.md`. Unqualified slugs (e.g., `hooks-pattern.md`) are the baseline/canonical entry for a `(feature, layer)` pair. Qualified slugs (e.g., `hooks-observability-pattern.md`) cover specific sub-patterns without displacing the baseline. `feature-matcher` prefers the baseline when the brief does not specifically justify a qualified variant; it may propose multiple supporting skills when they cover non-overlapping aspects. Slug collisions with approved skills are a hard error — skill-factory drafts must qualify or revise instead. **v2.3.2:** `skill-drafter` performs a pre-write check against `{catalog_root}/<slug>.md` and prepends a warning block to its confirmation when the proposed slug would collide, suggesting a qualified slug derived from the concept handle. The draft still writes to `.drafts/` — the warning exists so users see the overwrite risk before running `mv`.
 
 ### Hallucination gate