feat: patterns/ directory + skill-impact tracker (WikiSkill-inspired)

- New patterns/ directory with 7 initial failure-mode patterns (PAT-001..007)
- skill-impact.md audit trail for skill modifications
- _template.md for future pattern creation
- index.md updated with Patterns section
- log.md entry for this change
- Inspired by arXiv:2608.27454 (WikiSkill)

patterns/skill-impact.md

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+# Skill Impact Tracker
+
+> Inspired by WikiSkill (arXiv:2608.27454) — tracks skill modifications, their validation
+> outcomes, and acceptance decisions. Prevents repeating failed modifications and provides
+> an audit trail of skill evolution.
+
+## How to Use
+
+When a skill is patched, created, or deleted, append an entry to the table below.
+The entry records WHAT changed, WHY, and WHETHER it helped (if validated).
+
+### Entry Format
+
+```
+| Date | Skill | Change Type | Description | Validation | Outcome | Ref |
+```
+
+- **Change Type**: `created` | `patched` | `deleted` | `adopted`
+- **Validation**: How was success measured? (`manual` | `tested` | `benchmark` | `n/a`)
+- **Outcome**: `accepted` | `rejected` | `rolled-back` | `pending`
+- **Ref**: Session ID or solution doc path
+
+## Audit Trail
+
+| Date | Skill | Change Type | Description | Validation | Outcome | Ref |
+|------|-------|-------------|-------------|------------|---------|-----|
+| 2026-08-30 | compound-learning | patched | Added WikiMaintainer phase + patterns/ guidance from WikiSkill paper analysis | tested | accepted | this session |
+| 2026-08-13 | hindsight | patched | Expanded meta-memory automation table (decay scoring, drift detection, skill health) | tested | accepted | session 2026-08-13 |
+| 2026-08-13 | (3 cron jobs) | created | Decay scoring, drift detection, skill health check — no_agent cron jobs | tested | accepted | session 2026-08-13 |
+
+## Notes
+
+- **Rejected proposals persist here** — they are NOT deleted. Future skill updates can
+  consult this log to avoid repeating failed approaches (the WikiSkill paper showed this
+  is critical for effective evolution).
+- **Neutral changes** (no improvement, no regression) are recorded as `accepted` with
+  `validation: manual` — they may enable future improvements.
+- **Rollbacks** are explicitly tracked — if a skill patch caused regressions and was
+  reverted, the entry stays with `outcome: rolled-back`.