feat: add resize_pty protocol message for remote PTY resize

[willwashburn]

Summary

• Adds resize_pty protocol message across the full stack: Rust protocol types, broker dispatch, PTY worker handler, and TypeScript SDK client/types
• When PTY agents are spawned by SDK clients (e.g. Electron apps embedding xterm.js), the worker process has no terminal attached so SIGWINCH-based resize never fires — the PTY stays at its default 80x24
• New resizePty(name, rows, cols) SDK method lets clients send terminal dimensions to the broker, which forwards them to the worker, which calls pty.resize()

Changes

• src/protocol.rs: Add ResizePty { rows, cols } to BrokerToWorker enum
• src/main.rs: Add resize_pty handler in broker dispatch — validates agent exists, forwards to worker via send_to_worker, returns confirmation
• src/pty_worker.rs: Handle resize_pty message in worker select loop — calls pty.resize(rows, cols)
• packages/sdk/src/protocol.ts: Add resize_pty to SdkToBroker and BrokerToWorker types
• packages/sdk/src/client.ts: Add resizePty() method to AgentRelayClient

Test plan

• cargo check passes
• tsc --noEmit passes
• Existing protocol round-trip tests pass
• Manual test: spawn a PTY agent from an Electron app, call resizePty() on xterm resize, verify the agent TUI re-renders at the correct dimensions

🤖 Generated with Claude Code

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.

[barryonthecape]

Nice addition overall — wiring resize_pty through SDK → broker → worker is the right approach. I'm requesting changes for one behavior that can cause hard-to-debug terminal state issues.

Blocking issue:

• In src/pty_worker.rs, the new resize_pty handler silently falls back to rows=24 / cols=80 when payload fields are missing or invalid:
  • frame.payload.get("rows")...unwrap_or(24)
  • frame.payload.get("cols")...unwrap_or(80)

Why this is risky:

• A malformed frame can unexpectedly snap an active session back to 80x24 instead of failing fast.
• The broker already deserializes strongly typed u16 values for resize_pty; introducing permissive defaults in the worker creates inconsistent validation across layers.
• This can hide protocol/client bugs and make UI resize issues intermittent.

Requested fix:

1. Parse rows/cols strictly in the worker path (or decode as BrokerToWorker::ResizePty), and reject invalid payloads instead of defaulting.
2. Validate non-zero dimensions (rows > 0 && cols > 0) before calling pty.resize.
3. Return or emit a structured worker error for invalid resize payloads so clients can surface actionable diagnostics.

After that, this looks good to me.

[willwashburn]

@barryonthecape All three points addressed in 511cff4:

1. Strict deserialization — worker now uses `serde_json::from_value::` instead of manual field extraction with silent defaults. Invalid payloads return a `worker_error` with `invalid_payload` code.
2. Non-zero validation — both broker and worker reject `rows == 0 || cols == 0` with an `invalid_dimensions` error.
3. Structured error responses — worker sends typed `worker_error` frames for both invalid payloads and invalid dimensions so clients get actionable diagnostics.

Also added: broker rejects `resize_pty` for headless agents with `unsupported_operation`, and a round-trip test for the wire format.

[barryonthecape]

Thanks for the fast follow-up — requested changes are addressed.\n\nVerified on latest commits:\n- Worker now strictly deserializes resize payloads (no silent 80x24 fallback) and returns structured worker_error invalid_payload on malformed input.\n- Broker and worker both reject zero dimensions with explicit invalid_dimensions errors.\n- Broker now rejects resize_pty for non-PTY agents with unsupported_operation.\n- Added protocol round-trip coverage for BrokerToWorker::ResizePty wire format.\n\nLGTM.