feat: remote agent control (v1 up-lane)

[guohai]

Summary

Implements the Astation side of Remote Agent Control v1 per docs/specs/2026-05-28-remote-agent-control-design.md. Astation acts as a remote control that sends text and control keys to the focused Atem's agent over the existing channel (direct or relay). Voice reuses the existing PTT/hands-free path unchanged.

What's new

• AstationMessage.agentInput(agentId:kind:text:key:) — new message case with encode/decode. Matches the wire contract: atem_id stays in the relay envelope; the payload carries agentId + kind + text/key, with nil fields omitted.
• HubManager.sendAgentText / sendAgentKey — mirror sendVoiceCommand, route through routeToFocusedAtem() + sendHandler.
• RemoteControlWindowController — panel with a text field + Send and a key bar (Enter / Esc / Ctrl-C / ↑ / ↓ / y / n). Shows the target Atem; warns when none connected.
• Menu item "🎮 Remote Agent Control" (⌘R) in the status bar.

Wire contract

{ "atem_id": "<host>", "payload": {
  "type": "agentInput",
  "data": { "agentId": null, "kind": "text"|"key", "text": "...", "key": "enter|esc|ctrl-c|up|down|y|n" }
}}

Not in this PR

• Atem side (AstationMessage::AgentInput + PTY write) — separate repo; not yet implemented there, so the round-trip acceptance test can't pass until the Atem counterpart lands.
• agentId selection UI (v1 leaves it nil → Atem targets its focused/only agent).
• Screen mirroring / down-lane (later phase, mobile).

Test plan

• swift build succeeds
• 124 tests pass (was 119)
  • AgentInputMessageTests (5): text/key encode-decode, nil-field omission, round-trip
  • AgentInputRoutingTests (5): routes to connected Atem, correct payload per kind, explicit agentId, drop-when-no-Atem, pinned-client preference
• End-to-end requires the Atem-side AgentInput handler

Also fixes a stale RTCJoinOptions test that still expected aes256Gcm2 after the encryption default changed to .none.

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