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/galera-ddl-deadlock.md

@@ -0,0 +1,52 @@
+---
+pattern_id: PAT-001
+title: "OPTIMIZE TABLE on Galera triggers TOI DDL → Deadlock"
+category: database
+severity: high
+status: active
+first_observed: 2026-07
+last_updated: 2026-08-30
+related_systems: [galera-maxscale]
+related_solution_docs:
+  - docs/solutions/bug-fixes/2026-07-23-cnpg-timeline-corruption-rebuild.md
+related_skills: []
+---
+
+# OPTIMIZE TABLE on Galera triggers TOI DDL → Deadlock
+
+## Symptom
+
+Running `OPTIMIZE TABLE` on a Galera cluster node causes a cluster-wide stall or deadlock.
+The table lock propagates via TOI (Total Order Isolation) to all nodes, blocking all writes
+to ALL tables during the operation.
+
+## Root Cause
+
+Galera executes DDL statements (including `OPTIMIZE TABLE`, `ALTER TABLE`, etc.) in TOI mode.
+This means the DDL is replicated as a global operation that blocks the entire cluster — not
+just the target table. For large tables, the rebuild phase can take minutes, causing apparent
+outages.
+
+## Mitigation
+
+1. **Use RSU before DDL**: Switch the node to Rolling Schema Update (RSU) mode before running
+   `OPTIMIZE TABLE`. This prevents cluster-wide blocking:
+   ```
+   SET GLOBAL wsrep_OSU_method = 'RSU';
+   -- run OPTIMIZE TABLE on this node only
+   SET GLOBAL wsrep_OSU_method = 'TOI';  -- restore
+   ```
+2. **Schedule during maintenance window** — even with RSU, the node itself is degraded
+3. **Consider pt-online-schema-change** for large tables — avoids blocking entirely
+
+## Prevention
+
+- Never run `OPTIMIZE TABLE` or `ALTER TABLE` in production without RSU on Galera
+- Add this check to DBA runbooks and monitoring alerts
+- The Galera skill (`mariadb-galera-cluster-administration`) documents this pitfall
+
+## Evidence
+
+- Observed during Galera cluster administration sessions (Jul 2026)
+- MEMORY.md entry: "OPTIMIZE TABLE=TOI DDL→Deadlock! RSU davor."
+- Galera cluster: nodes 300/301/302, VIP .70:3306