Improve header-line: activity phases, git branch, layout redesign

[dnouri]

Summary

Replace the braille spinner with semantic activity labels, reorganize the header-line layout, add git branch display, and support setWorkingMessage from extensions.

• Replace Spinner with Semantic Activity Phases
• Header-Line Redesign — Layout, Git Branch, Working Message

Changes

1. Activity phases: Show thinking/replying/running/compact/idle instead of animating braille dots. Fixed 8-char slot prevents layout jumping. Event-driven phase transitions replace the global 100ms spinner timer.

2. Header-line layout redesign: Reorganize into four collapsing groups separated by │ — each pipe only appears when its group has content:

   [model • thinking activity] │ [stats] │ [branch · session] │ [ext-status · working-msg]
   

3. Git branch: Show current branch via vc-git--symbolic-ref, cached and refreshed on agent_end. Grouped with session name as project context.

4. setWorkingMessage handler: Extensions can show transient activity messages (e.g., "📖 Skimming the source…") in the header-line. Handler is ready; waiting on upstream pi-mono PR to emit the event over RPC.

References

• feat: support setWorkingMessage in RPC mode #111 — feature request for setWorkingMessage support
• Detailed TODO with phasing, acceptance criteria, and design rationale attached as a comment below

[dnouri]

As per discussion with @SayreBlades, we'll get rid of the Git branch name in the header-line.

Unclear if we wanna replace the braille spinner with activity only (e.g. thinking)? There'd be no real animation for when Pi is busy, but then it would be easy enough to spot the idle activity and know that it's not doing any work.

[dnouri]

This TODO file should be useful for the implementation. However it doesn't contain the updated requirement with regard to Git branch name, for instance: TODO-improve-header-line.md