feat(opencode): add opencode integration — /mcpctl switcher + footer indicator

Implements mcpctl as an opencode addon, mirroring the prime-agent UX
(active-project indicator + a /mcpctl project switcher). opencode is a
native MCP client, so instead of a bridge this mounts the active project's
proxy MCP gateway through opencode's own live MCP API.

- src/opencode-ext/mcpctl-opencode.ts — server plugin (headless runs): mounts
  the active project under a stable `mcpctl` name via `client.mcp.add`,
  re-asserting on first contact and before each turn; never throws.
- src/opencode-ext/mcpctl-opencode-tui.tsx — TUI plugin: `/mcpctl`,
  `/mcpctl-status`, `/mcpctl-skills`, and a `mcpctl:<project>` indicator in
  the prompt footer (next to the model name, above the token counter); the
  switch delegates to the CLI then re-points the mount live.
- `mcpctl config opencode` — provision/reuse the project mcptoken, install +
  register the plugins (tui.json), write the 0600 state file, sync skills.
- `skills sync --agent opencode` — new shared-tree target
  (~/.config/opencode/skill).
- embedded-source generator + embed test, settings/config/order tests,
  completions, README + docs/opencode-extension.md.

Typechecked against the real @opencode-ai/plugin types (1.18.15).

README.md

@@ -195,6 +195,51 @@ Inside pi:
 
 See [docs/pi-extension.md](docs/pi-extension.md) for full details.
 
+### Connect opencode
+
+[opencode](https://opencode.ai) is a native MCP client, so mcpctl ships an
+opencode integration that mounts the active project's proxy MCP gateway as an
+MCP server — no extra MCP library, and no `~/.claude` dependency.
+
+```bash
+mcpctl config opencode --project monitoring
+```
+
+This installs two plugins into `~/.config/opencode/`:
+
+- **server plugin** (`plugin/mcpctl.ts`) — auto-discovered; mounts the active
+  project's MCP gateway through opencode's own live MCP API, so headless runs
+  (`opencode run ...`) see the project's tools too;
+- **TUI plugin** (`mcpctl/mcpctl-tui.tsx`, registered in `tui.json`) — the
+  `/mcpctl` switcher and a `mcpctl:<project>` indicator in the prompt footer.
+
+It also mints/reuses the project's bearer token (stored 0600 in
+`~/.mcpctl/opencode-state.json`), and runs an initial skills sync into
+`~/.config/opencode/skill/`.
+
+Inside opencode:
+
+- `/mcpctl` — **switch project** from a filterable picker
+- `/mcpctl-status` — show the active project + MCP mount state
+- `/mcpctl-skills` — re-sync this project's skills into opencode's skill dir
+- the active project shows up as `mcpctl:<project>` next to the model name,
+  just above the token counter — switch it without restarting opencode
+
+Options (mirror the prime-agent command):
+
+```bash
+mcpctl config opencode --project monitoring --token mcpctl_pat_xxx  # provide token, don't mint
+mcpctl config opencode --project monitoring --skip-skills            # don't sync skills
+mcpctl config opencode --project monitoring --skip-plugin            # don't (re)install the plugins
+mcpctl config opencode --project monitoring --skip-marker            # don't touch .mcpctl-project here
+mcpctl config opencode --project monitoring --opencode-dir /path     # custom opencode config dir
+mcpctl config opencode --project monitoring --dry-run                # preview only
+```
+
+The `/mcpctl` switcher runs with `--skip-plugin --skip-marker`, so switching
+projects from inside opencode never re-scopes whichever repository opencode
+happened to be started in.
+
 ## Declarative Configuration
 
 Everything can be defined in YAML and applied with `mcpctl apply`: