effectify Pty service

[kitlangton]

Summary

• Migrate Pty from Instance.state() to Effect service pattern
• State uses Map<PtyID, ActiveSession> in layer closure
• Instance.bind replaced by InstanceContext closure capture
• Effect.addFinalizer handles process cleanup on instance disposal
• Sync facades (list, get, resize, write, connect) use runSyncInstance
• Async facades (create, update, remove) use runPromiseInstance
• Adds runSyncInstance helper to runtime.ts

Test plan

• bun run typecheck passes
• Verify PTY creation/connection works in TUI