feat(fullscreen): add extension workflows and session compaction

[shuyhere]

Summary

• add fullscreen-native extension workflows: menus, prompts, hidden dispatch turns, and direct saved-agent activation
• add manual and automatic session compaction with queued-prompt handling and turn-runner recovery support
• improve fullscreen transcript UX with block focus/copy, queued-message editing, compaction rendering, and related runtime updates

Why

This groups the remaining fullscreen/controller/runtime work into one stacked PR on top of the already-separated extension and skill-management foundations.

Includes

• fullscreen controller/runtime queue handling
• extension command UI integration
• hidden dispatch prompts
• manual + auto compaction
• transcript compaction rendering and interactions
• focused tests for fullscreen and turn-runner behavior

Validation

• cargo test -p bb-cli fullscreen::session::tests --quiet
• cargo test -p bb-cli turn_runner::tests --quiet
• cargo test -p bb-tui fullscreen::tests::interaction --quiet
• cargo test -p bb-tui fullscreen::projection::tests --quiet
• cargo test -p bb-tui fullscreen::frame::tests --quiet