feat: Support displaying combo and subagent transcripts

[linw1995]

Closes #129

Summary

This PR adds support for displaying and navigating transcripts from combo and subagent executions, providing users with better visibility into the execution history and conversation flow.

Changes

Core Features

• Transcript Storage: Added storage for combo and subagent transcripts in the chat state, persisting across sessions
• Multi-level Transcript Navigation: Implemented a scope-based navigation system allowing users to drill down from root transcript to combo/subagent transcript
• Transcript Links: Tool uses and results in transcripts now display as actionable links when they have associated combo/subagent executions
• Breadcrumb Navigation: Added breadcrumb display showing current transcript scope (e.g., Transcript / Root or Transcript / Combo: script_name)

UI/UX Improvements

• Keyboard Shortcuts:
  • Enter - Open linked transcript (when on a tool use/result with a combo/subagent)
  • Esc or Backspace - Go back to previous scope or close transcript
  • k/j or Up/Down - Navigate between transcript messages
• Visual Indicators: Actionable items show Open hint in shortcut hints
• Dynamic Labels: Back button shows Back when in nested scope, Close when at root

Technical Implementation

• Added TranscriptScope enum to track navigation state (Combo/Subagent with id and name)
• Added ComboTranscript and SubagentTranscript structs for serialization
• Implemented build_transcript_link_map() to identify tool uses with associated executions
• Refactored execute_subagent() to use a SubagentExecutionParams struct for cleaner code organization
• Added emit_combo_transcript() and emit_subagent_transcript() to capture execution history

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 436 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.48%. Comparing base (a46f02c) to head (0fa358d).

Files with missing lines	Patch %	Lines
crates/coco-tui/src/components/chat.rs	0.00%	251 Missing ⚠️
crates/coco-tui/src/components/transcript.rs	0.00%	119 Missing ⚠️
src/tools/run_task.rs	0.00%	41 Missing ⚠️
src/tools/run_combo.rs	0.00%	16 Missing ⚠️
crates/coco-tui/src/components/messages.rs	0.00%	5 Missing ⚠️
crates/coco-tui/src/components/message.rs	0.00%	3 Missing ⚠️
.../coco-tui/src/components/messages/tool/run_task.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #130      +/-   ##
==========================================
- Coverage   50.22%   49.48%   -0.75%     
==========================================
  Files          93       93              
  Lines       25033    25406     +373     
==========================================
- Hits        12574    12572       -2     
- Misses      12459    12834     +375     

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