refactor: migrate library crates from anyhow to thiserror

[bug-ops]

Summary

• Replace anyhow with typed thiserror error enums in all 7 library crates
• LlmError (11 variants), MemoryError (7), SkillError (9), IndexError (8), ChannelError (2), McpError (+4), TuiError (3)
• LlmProvider trait now returns Result<T, LlmError> instead of anyhow::Result
• Box<QdrantError> used where clippy flags large error size
• zeph-core stays on anyhow — typed errors auto-convert via ?
• 54 files changed, 6 new error.rs files

Test plan

• cargo nextest run --workspace --lib --bins — 1347 passed
• cargo clippy --workspace -- -D warnings — zero warnings
• Code review: approved (0 blockers)
• Verified zero anyhow in library crate sources

Phase 2 of #268. Closes #270.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 53.72340% with 261 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/zeph-llm/src/candle_provider/embed.rs	0.00%	35 Missing ⚠️
crates/zeph-memory/src/qdrant.rs	6.25%	30 Missing ⚠️
crates/zeph-index/src/store.rs	7.40%	25 Missing ⚠️
crates/zeph-skills/src/qdrant_matcher.rs	8.33%	22 Missing ⚠️
crates/zeph-llm/src/openai.rs	47.22%	19 Missing ⚠️
crates/zeph-llm/src/candle_provider/loader.rs	41.37%	17 Missing ⚠️
crates/zeph-mcp/src/registry.rs	37.50%	15 Missing ⚠️
crates/zeph-llm/src/candle_provider/mod.rs	7.69%	12 Missing ⚠️
crates/zeph-memory/src/semantic.rs	60.00%	12 Missing ⚠️
crates/zeph-index/src/watcher.rs	9.09%	10 Missing ⚠️
... and 16 more

❌ Your patch status has failed because the patch coverage (53.72%) is below the target coverage (60.00%). You can increase the patch coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #277      +/-   ##
==========================================
- Coverage   81.13%   81.03%   -0.11%     
==========================================
  Files          87       87              
  Lines       24556    24588      +32     
==========================================
  Hits        19924    19924              
- Misses       4632     4664      +32     

Files with missing lines	Coverage Δ
crates/zeph-index/src/chunker.rs	95.23% <100.00%> (ø)
crates/zeph-index/src/repo_map.rs	89.94% <100.00%> (ø)
crates/zeph-llm/src/any.rs	89.26% <100.00%> (ø)
crates/zeph-llm/src/orchestrator.rs	93.70% <100.00%> (+0.04%)	⬆️
crates/zeph-mcp/src/error.rs	100.00% <100.00%> (ø)
crates/zeph-skills/src/loader.rs	99.61% <100.00%> (+0.02%)	⬆️
crates/zeph-skills/src/matcher.rs	97.93% <100.00%> (ø)
crates/zeph-skills/src/registry.rs	91.25% <100.00%> (ø)
crates/zeph-skills/src/watcher.rs	92.40% <100.00%> (ø)
crates/zeph-tui/src/channel.rs	100.00% <100.00%> (ø)
... and 26 more

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