[E2-1] Session ownership model — any key can operate on any session

[OneStepAt4time]

Finding: SD-AUTHZ-01 (HIGH) | Milestone: M-E2: Identity & Access (#16)

Problem: Any API key can operate on any session regardless of which key created it.

Fix: Add createdByKeyId to SessionInfo. Enforce in sendMessage, approve, reject, kill, interrupt, escape, transcript, capture_pane. Reject 403 if caller keyId does not match.

Files: src/session.ts, src/server.ts, src/api-contracts.ts, src/mcp-server.ts

Acceptance: Key A cannot approve Key Bs session permission prompts; returns 403.