[Feature Request][Pi] Expose API to programmatically enter/exit planning mode #622

Problem

Third-party extensions cannot trigger plannotator's planning mode programmatically. The only ways to enter planning mode are:

• Manually typing /plannotator
• Using Ctrl+Alt+P

There is no API for another extension to call when it wants to put the agent into planning mode.

Use Case

When building persona-based workflows, different personas correspond to different phases of work. For example, a "plan" persona should automatically enter plannotator planning mode so that plannotator_submit_plan works immediately without requiring the user to manually type /plannotator first.

Current Behavior

1. Extension switches persona to "plan"
2. Agent gets plan-specific system prompt
3. Agent writes a plan and calls plannotator_submit_plan
4. Error: Not in plan mode. Use /plannotator to enter planning mode first.
5. User must manually intervene

Desired Behavior

Extensions should be able to trigger planning mode (and exit it) programmatically via Pi's ExtensionAPI.