fix(codex): align with the standard SessionDirTemplate pattern

Codex was the only agent that bind-mounted its host home into the
container via an explicit `Mounts: [{home}/.codex → /root/.codex]`
entry, while every other ACP agent uses `SessionConfig.SessionDirTemplate`
+ `SessionDirTarget`. Move codex onto the same surface so the
host-state isolation work in the next commit covers it uniformly.

This change alone does not stop the host-state leak — the bind mount
still resolves to the host's ~/.codex for now — but it removes the
last agent-specific branch in the runtime config so the docker
container manager can apply one rule for all agents.

apps/backend/internal/agent/agents/codex_acp.go

@@ -88,13 +88,19 @@ func (a *CodexACP) Runtime() *RuntimeConfig {
 		Env:         map[string]string{},
 		Mounts: []MountTemplate{
 			{Source: "{workspace}", Target: "/workspace"},
-			{Source: "{home}/.codex", Target: "/root/.codex"},
 		},
 		ResourceLimits: ResourceLimits{MemoryMB: 4096, CPUCores: 2.0, Timeout: time.Hour},
 		Protocol:       agent.ProtocolACP,
 		SessionConfig: SessionConfig{
 			NativeSessionResume: true,
 			CanRecover:          &canRecover,
+			// Use the same SessionDirTemplate pattern every other ACP agent
+			// uses; the docker container manager wires this into a kandev-owned
+			// per-container session dir, isolated from the host's ~/.codex
+			// (which carries host-absolute rollout paths in state.db that
+			// don't resolve inside the container).
+			SessionDirTemplate: "{home}/.codex",
+			SessionDirTarget:   "/root/.codex",
 		},
 	}
 }

apps/backend/internal/agent/agents/codex_acp_test.go

@@ -0,0 +1,29 @@
+package agents
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestCodexACPRuntimeNoLongerBindMountsHostHome(t *testing.T) {
+	a := NewCodexACP()
+	rt := a.Runtime()
+
+	for _, m := range rt.Mounts {
+		if strings.Contains(m.Source, "{home}") {
+			t.Fatalf("codex Mounts unexpectedly references {home}: %+v", m)
+		}
+	}
+}
+
+func TestCodexACPSessionDirTemplate(t *testing.T) {
+	a := NewCodexACP()
+	cfg := a.Runtime().SessionConfig
+
+	if cfg.SessionDirTemplate != "{home}/.codex" {
+		t.Fatalf("SessionDirTemplate = %q, want %q", cfg.SessionDirTemplate, "{home}/.codex")
+	}
+	if cfg.SessionDirTarget != "/root/.codex" {
+		t.Fatalf("SessionDirTarget = %q, want %q", cfg.SessionDirTarget, "/root/.codex")
+	}
+}