[P0] Prompt delivery verification — capture-pane after send-keys

[OneStepAt4time]

Problem

tmux send-keys is fire-and-forget. ~20% of prompts don't arrive because:

• Enter key race condition (500ms delay helps but doesn't guarantee)
• CC might be in a state that doesn't accept input (permission prompt, thinking)
• No way to verify the prompt actually appeared in CC's input

Expected Behavior

After sending a prompt via send-keys, Aegis should verify delivery by:

1. capture-pane after a short delay
2. Check if the sent text appears in the pane output
3. Retry if not found (up to 3 attempts with increasing delay)
4. Return delivery status in the API response

Files to modify

• src/tmux.ts — add verifyDelivery() method
• src/session.ts — use verification in sendMessage()
• src/server.ts — return delivery status in POST /send response

Acceptance Criteria

• After send-keys, capture-pane verifies the text appeared
• Retry logic (3 attempts, exponential backoff)
• API returns { delivered: true/false }
• Tests for verification logic
• Delivery rate >99% (from current ~80%)