feat: sticky session context — pin a one-line task note to the top of the session UI

Context

When working on long sessions, it's easy to lose track of what the current task or command is without scrolling back up. A pinned, always-visible summary at the top of the session pane would eliminate this friction.

Description

Add a way to pin a short (one-line) "current task" or session note that stays visible at the top of the Claude Code session UI throughout the conversation. This could be set via a slash command (e.g. /note My task: implement cohort apply flow) or a config setting, and cleared/updated at will.

The status line at the bottom already shows model/context info — this would be the complementary top anchor for user-defined session context.

Acceptance criteria

• [ ] User can set a short sticky note for the session via a slash command or settings key
• [ ] The note persists visibly at the top of the session without requiring scrolling
• [ ] The note can be updated or cleared during the session
• [ ] Works in both terminal and IDE extension surfaces (or gracefully degrades)

Notes

Related: the existing status line (/statusline) shows built-in variables but does not support free-form user text. This request is for a complementary, user-controlled anchor.