feat(backend): add opencode ACP (Agent Client Protocol) client

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.

Conversation: 019f354c-a51b-7ec3-83ad-c647e9b50b19

internal/acp/messages.go

@@ -0,0 +1,121 @@
+package acp
+
+import "encoding/json"
+
+// jsonRPCMessage is the raw JSON-RPC 2.0 envelope used over NDJSON.
+type jsonRPCMessage struct {
+	JSONRPC string          `json:"jsonrpc"`
+	ID      *int            `json:"id,omitempty"`
+	Method  string          `json:"method,omitempty"`
+	Params  json.RawMessage `json:"params,omitempty"`
+	Result  json.RawMessage `json:"result,omitempty"`
+	Error   *rpcError       `json:"error,omitempty"`
+}
+
+// rpcError is a JSON-RPC 2.0 error object.
+type rpcError struct {
+	Code    int    `json:"code"`
+	Message string `json:"message"`
+	Data    any    `json:"data,omitempty"`
+}
+
+// InitializeParams is sent once to negotiate the ACP session.
+type InitializeParams struct {
+	ProtocolVersion    int                `json:"protocolVersion"`
+	ClientCapabilities ClientCapabilities `json:"clientCapabilities"`
+	ClientInfo         ClientInfo         `json:"clientInfo"`
+}
+
+// ClientCapabilities describes tools the client exposes to the agent.
+type ClientCapabilities struct {
+	FS ClientFSCapabilities `json:"fs"`
+}
+
+// ClientFSCapabilities declares filesystem support.
+type ClientFSCapabilities struct {
+	ReadTextFile  bool `json:"readTextFile"`
+	WriteTextFile bool `json:"writeTextFile"`
+}
+
+// ClientInfo identifies this client to the agent.
+type ClientInfo struct {
+	Name    string `json:"name"`
+	Version string `json:"version"`
+}
+
+// InitializeResult is returned by the agent for initialize.
+type InitializeResult struct {
+	ProtocolVersion   int `json:"protocolVersion"`
+	AgentCapabilities any `json:"agentCapabilities"`
+	AgentInfo         any `json:"agentInfo"`
+}
+
+// SessionNewParams starts a new agent session for a workspace.
+type SessionNewParams struct {
+	Cwd        string `json:"cwd"`
+	MCPServers []any  `json:"mcpServers"`
+}
+
+// SessionNewResult returns the session identifier.
+type SessionNewResult struct {
+	SessionID string `json:"sessionId"`
+}
+
+// PromptMessage is a single user message part.
+type PromptMessage struct {
+	Type string `json:"type"`
+	Text string `json:"text"`
+}
+
+// SessionPromptParams sends user input to the agent.
+type SessionPromptParams struct {
+	SessionID string          `json:"sessionId"`
+	Prompt    []PromptMessage `json:"prompt"`
+}
+
+// SessionPromptResult is the final response for a prompt turn.
+type SessionPromptResult struct {
+	StopReason string `json:"stopReason"`
+}
+
+// SessionCancelParams interrupts an in-flight turn.
+type SessionCancelParams struct {
+	SessionID string `json:"sessionId"`
+}
+
+// SessionUpdateParams is the payload of a session/update notification.
+type SessionUpdateParams struct {
+	SessionID string `json:"sessionId"`
+	Update    Update `json:"update"`
+}
+
+// Update carries a single streamed update from the agent.
+type Update struct {
+	SessionUpdate string `json:"sessionUpdate"`
+	MessageID     string `json:"messageId,omitempty"`
+	Content       string `json:"content,omitempty"`
+	ToolCallID    string `json:"toolCallId,omitempty"`
+}
+
+// FSReadParams is sent by the agent to read a workspace file.
+type FSReadParams struct {
+	Path      string `json:"path"`
+	SessionID string `json:"sessionId"`
+	Line      *int   `json:"line,omitempty"`
+	Limit     *int   `json:"limit,omitempty"`
+}
+
+// FSReadResult returns file contents to the agent.
+type FSReadResult struct {
+	Content string `json:"content"`
+}
+
+// FSWriteParams is sent by the agent to write a workspace file.
+type FSWriteParams struct {
+	Path      string `json:"path"`
+	SessionID string `json:"sessionId"`
+	Content   string `json:"content"`
+}
+
+// FSWriteResult is the empty success response for fs/write_text_file.
+type FSWriteResult struct{}