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/process/manager.go

@@ -36,10 +36,10 @@ type LocalManager struct {
 
 // NewManager creates a LocalManager with the given command.
 // If command is empty, it defaults to "opencode".
-func NewManager(command string) *LocalManager {
+func NewManager(command string, args []string) *LocalManager {
 	return &LocalManager{
 		command:  normalizeCommand(command),
-		args:     []string{},
+		args:     args,
 		sessions: make(map[string]*Session),
 	}
 }
@@ -267,4 +267,3 @@ func (m *LocalManager) exitOf(workspaceID string) (ExitInfo, error) {
 	defer sess.mu.Unlock()
 	return sess.Exit, nil
 }
-