feat(tui): subagent optimization with composer integration and file suggestions

[shayne-snap]

Summary

Optimize the subagent experience in the TUI with improved composer integration, file path suggestions, and better delegation flows.

Changes

• file_suggestions.go — New component for inline file path completion in the composer
• composer/composer.go — Refactored to support subagent delegation and improved state management
• model.go / model_events.go / model_keys.go / model_prompt.go — Enhanced subagent interaction handling and keyboard shortcuts
• model_test.go — Comprehensive test coverage for subagent-related interactions (319 new lines)
• open_command.go — Updated to handle subagent-aware file opening
• history_nav.go — Improved navigation history for subagent turns
• render.go / review.go / skills.go / slash.go — Supporting updates across the TUI layer
• README updates — Document new subagent capabilities

Testing

• Added extensive unit tests in model_test.go
• Updated existing composer tests
• All existing tests pass

Closes #(issue)