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/tests/fixtures/skill-drafter/slug-collision-expected.md

@@ -0,0 +1,159 @@
+# Skill-drafter slug-collision — expected output fixture
+
+This fixture documents the expected confirmation-message shape from
+`skill-drafter` when it detects a slug collision at the catalog root.
+It is a reference fixture — not an auto-executed test — used to
+verify the agent by running a synthetic scenario by hand and
+comparing output against the shapes below.
+
+## Scenario A — no collision (baseline)
+
+### Synthetic input
+
+- **Catalog root:** `skills/cc-architect-catalog/`
+- **Concept JSON (from concept-extractor):**
+  ```json
+  {
+    "cc_feature": "worktrees",
+    "layer": "pattern",
+    "concept": "parallel feature isolation",
+    "description": "Isolate concurrent feature work in per-task worktrees to avoid git race conditions.",
+    "source_path": "/tmp/worktrees-notes.md",
+    "out_of_scope": false
+  }
+  ```
+- **Catalog state at time of run:** no `worktrees-pattern.md` present
+  at catalog root (only `worktrees-reference.md`).
+
+### Expected confirmation output
+
+```
+Drafted: /Users/…/skills/cc-architect-catalog/.drafts/worktrees-pattern.md
+Word count: 312
+Frontmatter: 9 fields, review_status=pending, ngram_overlap_score=null
+Collision: none
+Next: ip-hygiene-checker for IP scoring
+```
+
+No warning block. `Collision: none`.
+
+## Scenario B — approved collision (primary case)
+
+### Synthetic input
+
+- **Catalog root:** `skills/cc-architect-catalog/`
+- **Concept JSON:**
+  ```json
+  {
+    "cc_feature": "hooks",
+    "layer": "pattern",
+    "concept": "observability progressive alerts",
+    "description": "Progressive-alert observability pattern for hook-driven telemetry.",
+    "source_path": "/tmp/hooks-telemetry-notes.md",
+    "out_of_scope": false
+  }
+  ```
+- **Catalog state:** `hooks-pattern.md` exists with
+  `review_status: approved` (the v2.3.0 seed).
+
+### Expected confirmation output
+
+```
+⚠️  Slug collision at catalog root
+   Existing: skills/cc-architect-catalog/hooks-pattern.md (review_status=approved)
+   Draft:    skills/cc-architect-catalog/.drafts/hooks-pattern.md (this run)
+   Risk:     manual `mv` during promotion will silently overwrite the existing file.
+   Suggested qualified slug: hooks-observability-pattern.md
+   Reason for qualifier:     concept handle names "observability" as the named sub-pattern.
+   Action:                   rename the draft before `mv`, or revise the existing baseline.
+
+Drafted: /Users/…/skills/cc-architect-catalog/.drafts/hooks-pattern.md
+Word count: 287
+Frontmatter: 9 fields, review_status=pending, ngram_overlap_score=null
+Collision: approved
+Next: ip-hygiene-checker for IP scoring
+```
+
+The warning is prepended. The draft is still written — the collision
+check is a hint, not a block. `Collision: approved` appears in the
+confirmation for downstream-log parsers.
+
+## Scenario C — soft collision (pending existing)
+
+### Synthetic input
+
+- **Catalog state:** `subagents-pattern.md` exists with
+  `review_status: pending` (a prior draft that was `mv`-ed up without
+  the status being flipped, or a skill-factory auto-merged entry).
+
+### Expected confirmation output
+
+```
+⚠️  Slug collision at catalog root
+   Existing: skills/cc-architect-catalog/subagents-pattern.md (review_status=pending)
+   Draft:    skills/cc-architect-catalog/.drafts/subagents-pattern.md (this run)
+   Risk:     manual `mv` during promotion will silently overwrite the existing file.
+   Suggested qualified slug: subagents-delegation-pattern.md
+   Reason for qualifier:     concept handle centers on "delegation chains".
+   Action:                   rename the draft before `mv`, or revise the existing baseline.
+
+Drafted: /Users/…/skills/cc-architect-catalog/.drafts/subagents-pattern.md
+Word count: 256
+Frontmatter: 9 fields, review_status=pending, ngram_overlap_score=null
+Collision: pending
+Next: ip-hygiene-checker for IP scoring
+```
+
+`Collision: pending` signals a soft collision — the existing file is
+not necessarily a guaranteed baseline. Reviewer decides.
+
+## Scenario D — collision but no good qualifier
+
+### Synthetic input
+
+- **Concept handle:** `generic feature note` (no specific noun)
+- **Catalog state:** `skills-reference.md` exists (approved).
+
+### Expected confirmation output
+
+```
+⚠️  Slug collision at catalog root
+   Existing: skills/cc-architect-catalog/skills-reference.md (review_status=approved)
+   Draft:    skills/cc-architect-catalog/.drafts/skills-reference.md (this run)
+   Risk:     manual `mv` during promotion will silently overwrite the existing file.
+   Suggested qualified slug: none — reviewer must pick one manually.
+   Action:                   rename the draft before `mv`, or revise the existing baseline.
+
+Drafted: /Users/…/skills/cc-architect-catalog/.drafts/skills-reference.md
+Word count: 198
+Frontmatter: 9 fields, review_status=pending, ngram_overlap_score=null
+Collision: approved
+Next: ip-hygiene-checker for IP scoring
+```
+
+The fallback line replaces the "Suggested qualified slug" value.
+`Reason for qualifier` is omitted.
+
+## How to verify against this fixture
+
+This is not an auto-run test. To verify:
+
+1. Stage a synthetic concept JSON matching one of the scenarios above.
+2. Invoke `skill-drafter` via the `Agent` tool with the concept JSON,
+   source path, and catalog root.
+3. Capture the confirmation output.
+4. Compare structurally against the corresponding scenario. The exact
+   word counts and paths will differ; the warning block, collision
+   field, and suggested-slug line are the load-bearing shapes.
+
+If a future change alters the confirmation format, update this file
+alongside the agent prompt so the two remain in lockstep.
+
+## Why a reference fixture and not a unit test
+
+`skill-drafter` is a prompt-driven Sonnet agent. There is no in-repo
+harness that executes prompts deterministically, so a unit test is
+not practical. The reference fixture is the next-best anchor — it
+documents the expected shape for human verification and for
+downstream agents that parse the confirmation (e.g., the
+skill-author-orchestrator's summary phase).