feat: prompt delivery verification via capture-pane

[OneStepAt4time]

Fix for Issue #1 — Prompt delivery ~20% failure rate

Problem: tmux send-keys is fire-and-forget. ~20% of prompts don't arrive because of race conditions, CC not ready, or Enter key timing.

Solution: Verify → Retry

verifyDelivery(windowId, sentText) in tmux.ts

After sending, captures the pane and checks for delivery evidence:

1. CC is working (spinner/status line) → confirmed
2. CC is interactive (permission, plan, question) → confirmed
3. Sent text visible in pane (first 40 chars prefix match) → confirmed
4. CC is idle with no trace of input → FAILED
5. Unknown state → benefit of the doubt (may be transitioning)

sendKeysVerified(windowId, text, maxAttempts=3) in tmux.ts

• Sends text, waits, verifies
• Retries with escalating delays: 500ms, 1500ms, 3000ms
• Returns { delivered: boolean, attempts: number }

API change: POST /sessions/:id/send

Now returns:

{ "ok": true, "delivered": true, "attempts": 1 }

Backward compatible — ok: true still present.

Tests

• 16 new tests in prompt-delivery.test.ts
• Covers: state detection, text matching, retry logic, API response shape
• All 208 tests pass

Closes #1