Add a CLI for session notes

[jongio]

Session notes are useful in the TUI, but scripts and quick terminal workflows cannot inspect or update them without editing config.json by hand.

Acceptance criteria:

• dispatch notes lists sessions that have notes.
• dispatch notes --json prints a stable JSON shape.
• dispatch notes get <id>, set <id> <text>, and clear <id> work through the existing config file.
• Help text documents the command.