feat: add agent-device acp, a deterministic stdio ACP agent

[XeonBloomfield]

Summary

Adds agent-device acp: an Agent Client Protocol v1 agent over stdio, so ACP clients such as Zed can drive devices from their agent panel. Before: ACP editors could only reach agent-device indirectly, through whatever LLM agent they run and its MCP config. After: agent-device acp is a first-class agent surface — each prompt line is one agent-device command in CLI syntax, executed through the same AgentDeviceClient path as MCP tools, streamed back as ACP tool calls with daemon progress updates and inline screenshot images.

Design choices:

• Deterministic agent, no LLM: natural-language prompts are refused with guidance; the reasoning stays in the ACP client's own model. Keeps the zero-runtime-dependency policy — the protocol layer is hand-rolled in src/acp/, reusing the MCP server's newline-delimited JSON-RPC transport, payload queue, and error formatting (the shared pieces are now exported from src/mcp/ instead of duplicated).
• Prompt lines are tokenized with the replay-script tokenizer and parsed by the real CLI parser, so ACP prompts, replay scripts, and the CLI share one dialect. Target flags (--platform, --device, --udid, --session, --state-dir) are sticky within an ACP session.
• session/cancel is intercepted ahead of the serialized inbound queue so it takes effect between command lines of a running prompt. The command already in flight runs to completion — there is no abort seam in the daemon client; documented as a v1 limitation.
• session/new advertises all MCP-exposed commands via available_commands_update (response-before-notification ordering is pinned by test).

Docs: new ACP section in agent-setup.md with the Zed agent_servers config, README pointer, CHANGELOG entry, CONTEXT.md command-surface vocabulary update.

23 files touched (13 new: src/acp/ + tests). Scope stayed within the planned command-surface addition; the only changes outside it are exports of previously-private MCP transport/formatting helpers and the replay tokenizer.

Validation

Full unit bundle green (3296 tests / 366 files, including 27 new ACP tests covering the router contract, prompt parsing, sticky flags, cancellation, refusal, screenshot image attachment, and out-of-band cancel interception). check:quick, check:layering, fallow audit (0 dead code / 0 duplication after deduplicating against src/mcp/), and test:smoke all pass — including a new smoke-acp.test.ts that spawns the real stdio process and drives initialize → session/new → prompt refusal → clean shutdown. Live device evidence: through the packaged build (dist/src/internal/bin.js acp), an ACP session ran devices against a real daemon with an isolated state dir and streamed back actual inventory (iPhone 17 Pro simulator + host macOS target) as a completed tool_call_update; the daemon and state dir were cleaned up afterwards. Zed itself was not exercised — the wire contract was verified against the official ACP v1 JSON schema; a follow-up interactive Zed session is the remaining real-client check.

🤖 Generated with Claude Code

[thymikee]

Review finding: available_commands_update advertises agent-device commands as ACP available commands, but the prompt parser only accepts bare CLI lines. ACP slash-command usage sends the command as prompt text with a leading slash (for example /devices or /snapshot -i); on this branch those resolve as unknown commands because parsePromptCommandLine only strips optional agent-device before calling the CLI parser. Please either accept a leading / command prefix and add tests for /devices / /snapshot -i, or stop advertising available_commands_update until slash-form invocation is supported.

[XeonBloomfield]

I think this line within the guide can be reverted as the previous behavior still works!

[thymikee]

Ready for human review.

I rechecked #1132 at head 5b802f60 after the ACP follow-up commits. The prior blocker is addressed: advertised ACP commands now accept slash-form prompt lines (/devices, /snapshot -i) and tests cover that path in src/acp/__tests__/prompt-lines.test.ts. session/new now rejects relative cwd and malformed/missing mcpServers, with router coverage for invalid params. The docs were updated to explain ACP as a first-class deterministic interface while keeping examples CLI-shaped.

Checks: 15 successful; only expected skipped Bundle Size and deploy-preview jobs. Remaining residual risk is real-client Zed validation, but the stdio/protocol behavior now matches the ACP usage shape reviewed earlier.