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>

README.md

@@ -131,9 +131,6 @@ banger vm session logs <vm> planner --stream stderr
 banger vm session stop <vm> planner
 ```
 
-For ACP-aware host tooling: `banger vm acp <vm>` bridges stdio to
-guest `opencode acp` over SSH.
-
 ## Config
 
 Config lives at `~/.config/banger/config.toml`. All keys optional.
@@ -159,14 +156,14 @@ Host → guest file/directory copies, declared per-user in
 `~/.config/banger/config.toml`:
 
 ```toml
-[[file_sync]]
-host = "~/.local/share/opencode/auth.json"
-guest = "~/.local/share/opencode/auth.json"
-
 [[file_sync]]
 host = "~/.aws"          # whole directory, recursive
 guest = "~/.aws"
 
+[[file_sync]]
+host = "~/.config/gh/hosts.yml"
+guest = "~/.config/gh/hosts.yml"
+
 [[file_sync]]
 host = "~/bin/my-script"
 guest = "~/bin/my-script"