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/webui/server_test.go

@@ -171,7 +171,7 @@ func TestVMShowPageRendersRunningActions(t *testing.T) {
 		ports: api.VMPortsResult{
 			Name: "smth",
 			Ports: []api.VMPort{
-				{Proto: "tcp", Port: 4096, Endpoint: "http://172.16.0.2:4096", Process: "opencode"},
+				{Proto: "tcp", Port: 4096, Endpoint: "http://172.16.0.2:4096", Process: "devserver"},
 			},
 		},
 	}
@@ -189,7 +189,7 @@ func TestVMShowPageRendersRunningActions(t *testing.T) {
 			t.Fatalf("body missing %q\n%s", want, body)
 		}
 	}
-	for _, unwanted := range []string{"opencode attach", "root@172.16.0.2"} {
+	for _, unwanted := range []string{"root@172.16.0.2"} {
 		if strings.Contains(body, unwanted) {
 			t.Fatalf("body unexpectedly contains %q\n%s", unwanted, body)
 		}