test(ultraplan-local): add skill-factory calibration fixtures

3 source/draft pairs that pin n-gram-overlap verdicts to representative prose:
- accepted (containment 0.014 / longestRun 3)
- needs-review (containment 0.211 / longestRun 12)
- rejected (containment 0.676 / longestRun 74)

Topics: session-start hooks, subagent delegation, output styles — proximate to
the production source material the skill-factory will ingest.

Plan: .claude/projects/2026-04-18-skill-factory-fase-1-mvp/plan.md (step 5)

plugins/ultraplan-local/tests/fixtures/skill-factory/README.md

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+# Skill-factory calibration fixtures
+
+These fixtures calibrate the IP-hygiene thresholds used by `scripts/ngram-overlap.mjs`. Each
+pair (`source-*`, `draft-*`) is hand-tuned so that the n-gram containment verdict lands in a
+specific band, anchoring the empirical thresholds against representative prose.
+
+## Pairs
+
+| Pair | Target verdict | Containment | Longest run | Notes |
+|------|----------------|-------------|-------------|-------|
+| `source-accepted.md` ↔ `draft-accepted.md` | **accepted** | 0.014 | 3 | Heavy paraphrase; concept-equivalent without phrase reuse |
+| `source-needs-review.md` ↔ `draft-needs-review.md` | **needs-review** | 0.211 | 12 | Mixed: paraphrased frame, retained domain phrasing |
+| `source-rejected.md` ↔ `draft-rejected.md` | **rejected** | 0.676 | 74 | Light edit on top of source; verbatim runs survive |
+
+## Verdict bands
+
+The verdict bands match the constants in `scripts/ngram-overlap.mjs`:
+
+- **accepted** — containment < 0.15 AND longestRun < 8
+- **needs-review** — between accepted and rejected
+- **rejected** — containment ≥ 0.35 OR longestRun ≥ 15
+
+If you change the thresholds in `ngram-overlap.mjs`, re-verify each fixture pair to confirm
+the calibration still holds. The fixtures are content-stable; the thresholds are the variable.
+
+## Why these topics
+
+The fixtures use Claude Code reference prose (session-start hooks, subagent delegation, output
+styles) so they live near the kind of source material the skill-factory will actually paraphrase
+in production. Drift between fixture domain and production domain would weaken the calibration
+signal.
+
+## Regeneration
+
+These files are committed to the repo as ground-truth fixtures. Do not regenerate them ad-hoc —
+edit deliberately, re-run the verification commands listed in `plan.md` Step 5, and commit
+intentionally.
+
+```bash
+node scripts/ngram-overlap.mjs tests/fixtures/skill-factory/draft-accepted.md \
+  tests/fixtures/skill-factory/source-accepted.md
+```