[Refactor] Split session CLI modules

[samzong]

What's changed?

• Split CLI parsing and dispatch out of main.rs, leaving it as a small startup shell.
• Moved command runner glue into focused modules for info, search, export, import, usage, sync, share init, and the TUI runner.
• Reused the export record schema for session JSON output and restored sync behavior regression coverage.

Why

• Reduce session CLI structural debt while preserving existing command behavior and verification coverage.