feat: graceful shutdown on Ctrl-C with MCP cleanup

[bug-ops]

Summary

• Add Agent::shutdown() with farewell message and MCP server cleanup
• Cancel-aware LLM streaming via tokio::select! on shutdown signal
• Native tool loop checks shutdown flag each iteration
• McpManager::shutdown_all_shared(&self) with 5s per-client timeout
• Shutdown called in all exit paths (CLI, TUI agent, TUI task)

Closes #355, closes #356, closes #357, closes #358

Test plan

• cargo nextest run --workspace --lib --bins (1427 pass)
• cargo clippy --workspace -- -D warnings (clean)
• cargo +nightly fmt --check (clean)
• Manual: start long response, Ctrl-C — verify "Shutting down..." and clean exit

[codecov-commenter]

Codecov Report

❌ Patch coverage is 17.24138% with 24 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/zeph-mcp/src/manager.rs	0.00%	8 Missing ⚠️
crates/zeph-core/src/agent/mod.rs	0.00%	7 Missing ⚠️
crates/zeph-core/src/agent/streaming.rs	45.45%	6 Missing ⚠️
src/main.rs	0.00%	3 Missing ⚠️

❌ Your patch status has failed because the patch coverage (17.24%) is below the target coverage (60.00%). You can increase the patch coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #359      +/-   ##
==========================================
- Coverage   79.39%   79.33%   -0.07%     
==========================================
  Files         100      100              
  Lines       25770    25796      +26     
==========================================
+ Hits        20460    20464       +4     
- Misses       5310     5332      +22     

Files with missing lines	Coverage Δ
src/main.rs	63.43% <0.00%> (-0.10%)	⬇️
crates/zeph-core/src/agent/streaming.rs	50.58% <45.45%> (-0.26%)	⬇️
crates/zeph-core/src/agent/mod.rs	81.55% <0.00%> (-0.43%)	⬇️
crates/zeph-mcp/src/manager.rs	82.39% <0.00%> (-2.09%)	⬇️

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