M10/Phase 3: Bundled skills and usage statistics

[bug-ops]

Epic: #74
Effort: XS (1 SP)

Deliverables

• New bundled skills: git, docker, api-request
• SQLite table: skill_usage (skill_name, invocation_count, last_used_at)
• Track which skills are injected per query
• /skills command shows all skills with usage statistics
• Skill stats available via SemanticMemory for context-aware matching

Acceptance Criteria

• 3 new bundled skills with proper SKILL.md format
• Usage statistics persisted in SQLite
• /skills command displays name, description, usage count, last used
• Unit tests for statistics tracking