refactor: improve session socket path management

[linw1995]

Summary

Refactor session socket path management to use global state instead of environment variables, preventing path loss when bash strips COCO_ prefixed environment variables.

Changes

• Centralize env var name: Add SESSION_SOCKET_ENV constant for consistent naming
• Global state management: Use global::session_socket_path() instead of environment variable passing
• Fix bash tool: Read path from global state, avoid env var stripping during execution
• Better tests: Add SessionSocketTestGuard helper to prevent env var pollution

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.43949% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.07%. Comparing base (55394f6) to head (3e8bb49).
⚠️ Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/combo/runner.rs	27.27%	16 Missing ⚠️
crates/coco-tui/src/combo_run_server.rs	0.00%	4 Missing ⚠️
crates/coco-tui/src/components/chat.rs	0.00%	2 Missing ⚠️
crates/coco-tui/src/main.rs	0.00%	1 Missing ⚠️
src/cmd/combo.rs	0.00%	1 Missing ⚠️
src/combo/session.rs	66.66%	1 Missing ⚠️
src/tools/bash.rs	98.46%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #177      +/-   ##
==========================================
- Coverage   48.12%   46.07%   -2.05%     
==========================================
  Files         105      108       +3     
  Lines       27916    30166    +2250     
==========================================
+ Hits        13435    13900     +465     
- Misses      14481    16266    +1785     

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