[P1] Session health check endpoint

[OneStepAt4time]

Problem

No API to check if a CC session is actually alive vs a zombie tmux window. Zeus/orchestrators can't distinguish between "CC is working" and "CC crashed silently."

Solution

Add GET /v1/sessions/:id/health that returns:

• tmux window exists? (pane check)
• Claude process running? (pgrep in pane)
• Last output timestamp
• Terminal state (idle/working/stalled)

Acceptance Criteria

• New endpoint GET /v1/sessions/:id/health
• Returns structured health info
• Works for active and dead sessions
• Tests