refactor: move combo execution into CLI command with session socket support

[linw1995]

Closes #124

Summary

This PR implements the "RunCombo: move execution into coco combo run" enhancement by migrating combo execution from the run_combo tool to a dedicated CLI command coco combo run with session socket integration.

Key Changes

1. New CLI Command (src/cmd/combo.rs)

• Added coco combo run <name> [args...] as the canonical entrypoint for combo execution
• Supports both standalone mode and inline mode when COCO_SESSION_SOCK is present
• Streams events and returns JSON summary

2. Session Socket Protocol Extensions (src/combo/session.rs)

• Extended session socket protocol to support combo run requests
• Added event streaming for real-time UI updates
• Implemented result aggregation and JSON output

3. TUI Bridge Components (crates/coco-tui/src/)

• combo_run_bridge.rs: Manages communication between CLI and TUI session
• combo_run_server.rs: Session socket server handling combo run events
• Updated chat.rs and messages/combo.rs: Enhanced to display streaming combo events

4. Tool Integration

• Modified bash tool to inject COCO_SESSION_SOCK when invoking coco combo run
• Agent now uses bash tool to execute combos via CLI instead of run_combo tool
• Preserved transcript linking for combo runs executed via bash tool

5. Permission & Safety

• Added permission handling for combo execution via Bash tool
• Ensures TTY or session socket availability before execution
• Proper error handling and user feedback

Behavior

• With running TUI: coco combo run demo a b streams events to the TUI and returns JSON summary
• Without TUI: Command starts the TUI and runs the combo
• Agent usage: Agent invokes coco combo run via bash tool, with proper environment injection

[codecov-commenter]

Codecov Report

❌ Patch coverage is 9.56873% with 1342 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.04%. Comparing base (a46f02c) to head (68e25cd).
⚠️ Report is 8 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/coco-tui/src/components/chat.rs	0.00%	591 Missing ⚠️
crates/coco-tui/src/components/messages/combo.rs	6.78%	206 Missing ⚠️
crates/coco-tui/src/combo_run_server.rs	0.00%	141 Missing ⚠️
src/cmd/combo.rs	0.00%	139 Missing ⚠️
src/tools/run_combo.rs	0.90%	110 Missing ⚠️
crates/coco-tui/src/combo_run_bridge.rs	0.00%	39 Missing ⚠️
crates/coco-tui/src/main.rs	35.18%	35 Missing ⚠️
src/agent.rs	0.00%	21 Missing ⚠️
src/combo/session.rs	85.85%	14 Missing ⚠️
crates/coco-tui/src/components/messages.rs	0.00%	10 Missing ⚠️
... and 6 more

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #150      +/-   ##
==========================================
- Coverage   50.22%   47.04%   -3.19%     
==========================================
  Files          93       98       +5     
  Lines       25033    27348    +2315     
==========================================
+ Hits        12574    12865     +291     
- Misses      12459    14483    +2024     

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