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/doc.go

@@ -58,7 +58,6 @@
 //	session_controller.go  guestSessionController, sessionRegistry
 //	ssh_client_config.go   daemon-managed SSH client key material
 //	workspace.go           ExportVMWorkspace, PrepareVMWorkspace
-//	opencode.go            opencode host-side helpers
 //
 // Host bootstrap (in this package):
 //