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_test.go

@@ -8,7 +8,7 @@ import (
 )
 
 func TestStatusBeforeStartIsNotRunning(t *testing.T) {
-	m := NewManager("opencode")
+	m := NewManager("opencode", nil)
 	s := m.Status("ws1")
 	if s.Running {
 		t.Error("expected Running=false before start")
@@ -19,7 +19,7 @@ func TestStatusBeforeStartIsNotRunning(t *testing.T) {
 }
 
 func TestStartRejectsDuplicateRunningSession(t *testing.T) {
-	m := NewManager("")
+	m := NewManager("", nil)
 	// Override command for testing using the test binary itself.
 	m.command = os.Args[0]
 	m.args = []string{"-test.run=TestHelperProcess", "--"}
@@ -43,7 +43,7 @@ func TestStartRejectsDuplicateRunningSession(t *testing.T) {
 }
 
 func TestStopMissingSessionIsNotFound(t *testing.T) {
-	m := NewManager("opencode")
+	m := NewManager("opencode", nil)
 	err := m.Stop("nonexistent")
 	if err == nil {
 		t.Fatal("expected error stopping missing session")