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/_README.md

@@ -0,0 +1,51 @@
+# Patterns Directory
+
+Inspired by [WikiSkill (arXiv:2608.27454)](https://arxiv.org/abs/2608.27454) — this directory
+contains structured **failure-mode patterns** and **successful strategies** extracted from
+real operational experience.
+
+## Purpose
+
+Unlike `systems/` (describes what exists) or `concepts/` (abstract conventions),
+`patterns/` captures **recurrent problems and their proven solutions** — the "lessons learned"
+that compound across incidents.
+
+## Structure
+
+Each pattern is a standalone Markdown file:
+
+```
+patterns/
+├── _README.md           ← this file
+├── _template.md         ← copy this for new patterns
+├── galera-ddl-deadlock.md
+├── traefik-reload-unreliable.md
+├── ...
+└── skill-impact.md      ← audit trail of skill modifications (accept/reject history)
+```
+
+## How Patterns Are Born
+
+1. **Incident occurs** → problem is diagnosed and fixed
+2. **Compound-learning cycle** runs → solution doc created in `~/docs/solutions/`
+3. **Pattern extracted** → if the failure mode is recurrent or broadly applicable,
+   a pattern page is created here
+4. **Wiki Maintainer** (currently manual, future: automated) consolidates evidence
+   from multiple incidents into the pattern page
+
+## Relationship to Other Layers
+
+| Layer | Holds | Retrieval |
+|-------|-------|-----------|
+| `~/docs/solutions/` | One-time detailed incident docs | `search_files` (keyword) |
+| `patterns/` (here) | Recurring failure-mode patterns | Browse `index.md`, cross-linked |
+| MEMORY.md (L0) | Compressed pointer if high-priority | System prompt injection |
+| Hindsight (L2) | Semantic index of all above | `hindsight_recall` |
+
+## Rules
+
+- **One pattern per file** — don't merge unrelated patterns
+- **Evidence-based** — cite real incidents (link to solution docs or session dates)
+- **Actionable** — every pattern must have a "Mitigation" or "Prevention" section
+- **Never delete without redirect** — if a pattern is obsolete, mark `status: superseded`
+  and link to the replacement