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

pkg/config/config.go

@@ -37,7 +37,8 @@ type WorkspaceConfig struct {
 
 // ProcessConfig holds OpenCode process settings.
 type ProcessConfig struct {
-	OpenCodeCommand string `yaml:"opencodeCommand"`
+	OpenCodeCommand string   `yaml:"opencodeCommand"`
+	Args            []string `yaml:"args"`
 }
 
 // ShellConfig holds interactive shell settings.
@@ -93,7 +94,7 @@ func Default() Config {
 			MaxHeaderBytes: 1 << 20, // 1 MiB
 		},
 		Workspace: WorkspaceConfig{Root: "./workspaces"},
-		Process:   ProcessConfig{OpenCodeCommand: "opencode"},
+		Process:   ProcessConfig{OpenCodeCommand: "opencode", Args: []string{"acp"}},
 		Shell:     ShellConfig{Command: "bash", Args: []string{"-i"}},
 		File:      FileConfig{MaxWriteBytes: 1 << 20},
 		Gin:       GinConfig{Mode: "release"},
@@ -150,6 +151,9 @@ func applyRaw(cfg *Config, raw rawConfig) error {
 	if raw.Process.OpenCodeCommand != "" {
 		cfg.Process.OpenCodeCommand = raw.Process.OpenCodeCommand
 	}
+	if raw.Process.Args != nil {
+		cfg.Process.Args = raw.Process.Args
+	}
 	cfg.Shell.Command = raw.Shell.Command
 	cfg.Shell.Args = raw.Shell.Args
 	if raw.File.MaxWriteBytes != 0 {
@@ -211,6 +215,9 @@ func applyEnv(cfg *Config) error {
 	if v := os.Getenv("CODESPACE_OPENCODE_COMMAND"); v != "" {
 		cfg.Process.OpenCodeCommand = v
 	}
+	if v := os.Getenv("CODESPACE_OPENCODE_ARGS"); v != "" {
+		cfg.Process.Args = strings.Split(v, ",")
+	}
 	if v := os.Getenv("CODESPACE_SHELL_COMMAND"); v != "" {
 		cfg.Shell.Command = v
 	}