refactor: restructure combo module and add streaming line buffering

[linw1995]

Changes

• Extract combo types: Move combo-related types to dedicated combo/types.rs module for better organization
• Restructure combo runner: Move run_combo from tools/ to combo/runner.rs for clearer module boundaries
• Add streaming line buffering: Introduce LineBuffer and BufferSet types to manage streaming text aggregation with support for line prefixes and multi-stream handling

Summary

This PR improves code organization by consolidating combo-related functionality into a dedicated module and enhances streaming output capabilities with line buffering support.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.12048% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.02%. Comparing base (c3970cb) to head (8180600).
⚠️ Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/agent/streaming.rs	82.91%	27 Missing ⚠️
src/combo/runner.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #164      +/-   ##
==========================================
+ Coverage   47.59%   48.02%   +0.43%     
==========================================
  Files         100      104       +4     
  Lines       27316    27579     +263     
==========================================
+ Hits        13000    13246     +246     
- Misses      14316    14333      +17     

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