Remove opencode package + vm acp command (dead code)

The `internal/opencode` package and the `opencodeCapability` that
consumed it were hard-wired to wait for opencode on guest port 4096
when an image shipped an initrd. After the prune commits (void /
alpine / customize.sh / image build all removed), nothing banger
produces today carries an initrd, so the capability's wait path was
unreachable: every startup short-circuited to the "direct-boot, skip
opencode" branch.

Same logic for `banger vm acp`: it SSHes to `opencode acp --cwd
<path>`, a binary the golden image no longer ships. Users who run
their own image with opencode can still invoke
`ssh vm -- opencode acp --cwd /root/repo` directly — no banger
scaffolding required.

Removed:
- internal/opencode/ (whole package, 255 LOC incl. tests)
- internal/daemon/opencode.go (opencodeCapability)
- cli `vm acp` command + its helpers (runVMACP, sshACPCommandArgs,
  vmACPRemoteCommand) + their tests
- The opencodeCapability{} entry in registeredCapabilities() plus
  the test that pinned its presence
- `wait_opencode` progress-stage label from the vm-create renderer
- Stale mentions in daemon/doc.go, README, and webui test fixtures

~480 lines gone, 12 added. `banger/internal` is now 25 packages
instead of 26.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/daemon/capabilities_test.go

@@ -144,13 +144,13 @@ func TestContributeHooksPopulateGuestAndMachineConfig(t *testing.T) {
 	}
 }
 
-func TestRegisteredCapabilitiesIncludeOpencode(t *testing.T) {
+func TestRegisteredCapabilitiesInOrder(t *testing.T) {
 	d := &Daemon{}
 	var names []string
 	for _, capability := range d.registeredCapabilities() {
 		names = append(names, capability.Name())
 	}
-	want := []string{"work-disk", "opencode", "dns", "nat"}
+	want := []string{"work-disk", "dns", "nat"}
 	if !reflect.DeepEqual(names, want) {
 		t.Fatalf("capabilities = %v, want %v", names, want)
 	}