feat(skills): implement M10 advanced skills

[bug-ops]

Summary

• Embedding-based skill matcher with cosine similarity selects top-K relevant skills per user query instead of injecting all skills into system prompt
• Filesystem watcher (notify + debouncer) detects SKILL.md changes and hot-reloads without restart
• Skill priority: earlier paths in config take precedence for same-name skills
• SQLite skill_usage table tracks invocation counts; /skills command displays statistics
• Three new bundled skills: git, docker, api-request

Changes

New files:

• crates/zeph-skills/src/matcher.rs -- SkillMatcher, cosine similarity, EmbedFuture callback type
• crates/zeph-skills/src/watcher.rs -- SkillWatcher with notify-debouncer-mini (500ms debounce)
• crates/zeph-memory/migrations/004_skill_usage.sql -- skill_usage table
• skills/{git,docker,api-request}/SKILL.md -- bundled skills

Modified:

• agent.rs -- dynamic per-query skill selection, reload handling, /skills command
• config.rs -- max_active_skills field + ZEPH_SKILLS_MAX_ACTIVE env override
• registry.rs -- priority dedup (HashSet), reload(), into_skills()
• prompt.rs -- generic Borrow for both &[Skill] and &[&Skill]
• sqlite.rs -- record_skill_usage (UPSERT), load_skill_usage
• main.rs -- matcher/watcher initialization, embed_fn closure wiring

Dependencies: notify 8, notify-debouncer-mini 0.7

Test plan

• 234 workspace tests pass (0 failures, 6 skipped)
• clippy clean (-D warnings)
• cargo +nightly fmt --check clean
• Security audit: no critical/high issues
• Code review: approved, no blocking issues
• Manual test: verify skill matching with Ollama embedding model
• Manual test: add SKILL.md while running, verify hot-reload
• Manual test: run /skills command

Closes #74, closes #75, closes #76, closes #77

[codecov-commenter]

Codecov Report

❌ Patch coverage is 67.20930% with 141 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/zeph-core/src/agent.rs	33.33%	66 Missing ⚠️
src/main.rs	0.00%	34 Missing ⚠️
crates/zeph-skills/src/watcher.rs	0.00%	32 Missing ⚠️
crates/zeph-skills/src/matcher.rs	95.86%	6 Missing ⚠️
crates/zeph-core/src/config.rs	62.50%	3 Missing ⚠️

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
- Coverage   77.06%   76.18%   -0.88%     
==========================================
  Files          20       22       +2     
  Lines        3662     4078     +416     
==========================================
+ Hits         2822     3107     +285     
- Misses        840      971     +131     

Files with missing lines	Coverage Δ
crates/zeph-memory/src/sqlite.rs	99.63% <100.00%> (+0.03%)	⬆️
crates/zeph-skills/src/loader.rs	98.64% <ø> (ø)
crates/zeph-skills/src/prompt.rs	100.00% <100.00%> (ø)
crates/zeph-skills/src/registry.rs	99.08% <100.00%> (+0.93%)	⬆️
crates/zeph-core/src/config.rs	96.46% <62.50%> (-0.82%)	⬇️
crates/zeph-skills/src/matcher.rs	95.86% <95.86%> (ø)
crates/zeph-skills/src/watcher.rs	0.00% <0.00%> (ø)
src/main.rs	0.00% <0.00%> (ø)
crates/zeph-core/src/agent.rs	49.45% <33.33%> (-9.44%)	⬇️

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