Adds a minimal but real ACP stack for the opencode process:
- pkg/config: process.args default ["acp"] (opencodeCommand still "opencode")
- internal/process: NewManager(command, args) — exec.Command uses args
- internal/acp (new): NDJSON transport + JSON-RPC client over the existing
process stdio. Implements initialize / session/new / session/prompt /
session/cancel. Serves fs/read_text_file and fs/write_text_file from the
workspace's fs.FileSystem. terminal/* requests get MethodNotFound.
- internal/service/acp_service: per-workspace Client + mutex; starts the
process on first prompt; transparently re-init on restart.
- internal/api/acp_handler: GET /acp/status, GET /acp/history,
POST /acp/prompt, POST /acp/cancel.
- internal/model/acp: API DTOs.
- internal/acp/client_test: NDJSON split-lines, request/response correlation,
notification dispatch, agent-initiated request handling (fs + terminal).
Existing process WS endpoint and Shell subsystem are unchanged.
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tao.chen