refactor: extract agent submodules and add TUI chat state types

[linw1995]

Overview

Refactor src/agent.rs by splitting it into focused submodules to improve code maintainability and testability.

Main Changes

Module Extraction

• src/agent/message.rs (301 lines) - Message type definitions
• src/agent/provider_selection.rs (220 lines) - Provider selection logic
• src/agent/streaming.rs (386 lines) - Streaming processing implementation
• src/prompt_reply.rs (119 lines) - Prompt reply handling
• src/agent.rs - Reduced from 924 to 86 lines

New TUI State Types

• crates/coco-tui/src/components/chat/state.rs (186 lines) - Chat component state management
  • ChatState - Session state (Ready/Processing)
  • Focus - UI focus management
  • ViewMode - View mode switching
  • CancellationGuard - Double-key exit protection

Impact

• Clearer code structure with single responsibility per module
• Better unit testing and future extensibility
• Centralized TUI state management for improved UI maintainability

[codecov-commenter]

Codecov Report

❌ Patch coverage is 65.76087% with 252 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.82%. Comparing base (c3970cb) to head (20a1acd).
⚠️ Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/agent/streaming.rs	55.83%	125 Missing ⚠️
src/agent/message.rs	76.60%	51 Missing ⚠️
src/agent/provider_selection.rs	72.78%	40 Missing ⚠️
src/prompt_reply.rs	67.60%	23 Missing ⚠️
src/agent.rs	23.52%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #163      +/-   ##
==========================================
+ Coverage   47.59%   47.82%   +0.23%     
==========================================
  Files         100      104       +4     
  Lines       27316    27421     +105     
==========================================
+ Hits        13000    13115     +115     
+ Misses      14316    14306      -10     

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