refactor: improve combo reply handling

[linw1995]

Simplify combo reply offload flow by using proper env injection and moving response handling to session server.

Changes:

• Add set_bash_env/remove_bash_env methods to inject env vars via bash tool instead of string manipulation
• Move combo reply response handling from TUI stdout parsing to session server socket mechanism
• Remove redundant error types and JSON parsing logic from TUI

Benefits:

• Cleaner separation of concerns: session server owns reply response handling
• More reliable env injection via bash tool API vs string concatenation
• Simpler TUI code with fewer error cases to manage

[codecov-commenter]

Codecov Report

❌ Patch coverage is 23.52941% with 65 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.73%. Comparing base (e5977ab) to head (87bee25).
⚠️ Report is 9 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/coco-tui/src/components/chat.rs	11.32%	47 Missing ⚠️
src/agent/executor.rs	20.00%	8 Missing ⚠️
src/agent.rs	0.00%	6 Missing ⚠️
src/combo/starter.rs	50.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #106      +/-   ##
==========================================
- Coverage   53.46%   51.73%   -1.73%     
==========================================
  Files          88       91       +3     
  Lines       21248    23373    +2125     
==========================================
+ Hits        11360    12092     +732     
- Misses       9888    11281    +1393     

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