Refine session UX, server lifecycle, and client protocol recovery

[wangtsiao]

Summary

• Desktop: add session composer with fork boundaries and keep-alive session shell; persist composer draft and selected model across sessions.
• Server: generate and persist server-side session titles; extend message editing, session fork, live session settings, and model-selection persistence.
• Client protocol: harden Native subscription replay and protocol mismatch recovery; consolidate client I/O and TUI worker event paths; polish desktop session activity handling.

Key Changes

• Desktop session UX: session composer, fork boundary divider, keep-alive session shell, context occupancy ring, sidebar activity rows, and protocol mismatch recovery with native client replay.
• Server session lifecycle: server-side title generation and persistence, session fork and rollback persistence, message editing roundtrip, live session settings updates, and model selection persistence.
• Client/TUI consolidation: remove legacy stdio/event paths, unify native client interaction, and streamline TUI worker session restore and subagent events.
• Docs/specs: update session settings, message editing, session data model, and title generation specs with traceability updates.

Tests

• Added tests for native client subscription replay, protocol mismatch recovery, chat composer persistence, data-model ref behavior, and model-selection persistence.
• Updated roundtrip, persistence, provider routing/failure, goal continuation, compaction, fork, rollback, and TUI/sandbox tests across the workspace.

[wangtsiao]

Superseded by 7df-lab#190 (PR should target upstream).