Add local notes for sessions

[jongio]

Problem

Favorites mark important sessions, but they do not explain why a session matters or what follow-up is needed. Users returning days later still need to reopen the preview and infer context from the conversation.

Proposed solution

Add a note field keyed by session ID in config, similar to hiddenSessions and favoriteSessions. Press m to edit a short note for the selected session. Show the note near the top of internal/tui/components/preview.go and add a small marker in internal/tui/components/sessionlist.go when a note exists.

User impact

Developers can annotate handoffs, blockers, and next steps without leaving Dispatch.

Acceptance criteria

• Pressing m opens an editor for the selected session note.
• Notes persist in the Dispatch config and are keyed by session ID.
• Sessions with notes show the note in the preview pane and a marker in the session list.

Notes

Complexity: M • Priority: 4