feat(opencode): add ACP stdio server for Zed + CLI switch

[josephschmitt]

Summary

• Adds an ACP-compliant JSON-RPC server over stdio so Zed can use opencode as a headless agent.
• Reuses existing session and tool pipelines; minimal-first implementation, ready to iterate on streaming.

What Changed

• New packages/opencode/src/acp/server.ts: stdio JSON-RPC 2.0 server for ACP.
• CLI: --acp flag and ZED_ACP env switch in packages/opencode/src/index.ts.
• Tests: packages/opencode/test/acp.test.ts covers initialize → complete → shutdown.
• Docs: packages/opencode/README.md updated with ACP usage and methods.

ACP Methods

• initialize, shutdown
• text/complete (non-streaming for now)
• tool/list, tool/run
• files/read, files/write
• fs/glob
• session/start, session/stop
• diagnostics/log

Tooling Surface

• Exposes a basic set via ToolRegistry: read, write, glob, bash, webfetch.

Usage

• Run from repo: bun run packages/opencode/src/index.ts --acp
• Or install and run: set ZED_ACP=1 and invoke opencode
• Zed settings: add an ACP provider that runs ["opencode", "--acp"] (or the bun dev command).

Testing

• E2E: bun test packages/opencode/test/acp.test.ts
• Manual: pipe JSON-RPC lines to bun run packages/opencode/src/index.ts --acp and observe responses.

Notes

• Headless; respects project cwd provided by Zed.
• No streaming yet (planned follow-up).
• No breaking changes to existing HTTP server or CLI.