feat(skills): extract ironclaw_skills crate and integrate with v2 engine

Extract the skills system into a standalone `ironclaw_skills` crate
(following the ironclaw_safety pattern) and wire it into the v2 engine
for deterministic skill selection, CodeAct code injection, and
confidence tracking.

**ironclaw_skills crate** (94 tests):
- Core types: SkillManifest, ActivationCriteria, LoadedSkill, SkillTrust
- V2 types: V2SkillMetadata, CodeSnippet, SkillMetrics, V2SkillSource
- Deterministic 4-phase selector (gating→scoring→budget→attenuation)
- apply_confidence_factor() for extracted skill scoring
- SKILL.md parser, validation/escaping, gating, registry, catalog
- Feature-gated: catalog (reqwest), registry (filesystem)

**Engine integration** (14 new tests):
- DocType::Skill with retrieval weight 0.45
- SkillSelector bridges MemoryDoc→LoadedSkill for shared scoring
- SkillTracker for usage/version/rollback confidence tracking
- System prompt injection via <skill> XML blocks
- CodeAct snippet injection via Monty NameLookup
- Skill extraction mission replaces playbook extraction
- ThreadManager.set_skill_selector() for runtime wiring

**Bridge + migration**:
- skill_migration.rs: v1 SKILL.md → v2 MemoryDoc (idempotent)
- init_engine() migrates v1 skills, builds SkillSelector
- src/skills/mod.rs → re-export shim

**E2E test** (tests/engine_v2_skill_codeact.rs):
- Full CodeAct loop: skill selected → LLM returns Python code →
  Monty executes http() → mock returns canned GitHub JSON →
  FINAL() terminates → thread completes with canned data
- GitHub SKILL.md in skills/github/ as reference implementation

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Cargo.lock

@@ -3628,6 +3628,7 @@ dependencies = [
  "ironclaw_common",
  "ironclaw_engine",
  "ironclaw_safety",
+ "ironclaw_skills",
  "json5",
  "libsql",
  "lru",
@@ -3698,6 +3699,7 @@ version = "0.1.0"
 dependencies = [
  "async-trait",
  "chrono",
+ "ironclaw_skills",
  "monty",
  "pretty_assertions",
  "serde",
@@ -3720,6 +3722,25 @@ dependencies = [
  "url",
 ]
 
+[[package]]
+name = "ironclaw_skills"
+version = "0.1.0"
+dependencies = [
+ "chrono",
+ "futures",
+ "regex",
+ "reqwest",
+ "serde",
+ "serde_json",
+ "serde_yml",
+ "sha2",
+ "tempfile",
+ "thiserror 2.0.18",
+ "tokio",
+ "tracing",
+ "urlencoding",
+]
+
 [[package]]
 name = "is-docker"
 version = "0.2.0"