Add inter-extension dictation events

[tkoenig]

Closes #6

Summary

• accept pi-transcribe:toggle on Pi's shared event bus and route it through the same capture toggle used by the keyboard shortcut
• emit pi-transcribe:state with only { state: "starting" | "recording" | "transcribing" | "idle" }
• bind the event listener to the active session context, clean it up on shutdown, and return to idle after cancellation or failure paths
• document the small inter-extension contract and add a focused lifecycle test

The integration is process-local and generic; it does not add an HTTP server or any Stream Deck-specific behavior to pi-transcribe. Existing shortcut behavior remains unchanged.

Testing

• npm test