feat: Real-time .md session transcript with offset-based polling

[Skippy-the-Magnificent-one]

Tee all session output to a .md file in real-time. Track file offset so API consumers can poll for only NEW content since last read.

Benefits:

• Persistent artifact of what the agent did
• VS Code split view: terminal on right, .md on left = scrollable history
• No websockets needed -- simple file-based streaming
• Works for both --print and interactive modes

Implementation:

• Tee stdout/stderr to ~/.monkeyproof/sessions/<id>/transcript.md
• Track lastReadOffset per consumer
• GET /sessions/:id/output?since=<offset> returns only new content
• File persists after session ends for review