feat(opencode): native opencode addon — /mcpctl switcher, live project switching, footer indicator

Implement mcpctl as an opencode addon mirroring the pi/prime-agent
integrations. Mounts the active project's MCP gateway through opencode's own
live MCP API, so switching projects needs no restart and opencode.json is
never touched (the bearer token stays in a 0600 state file).

- server plugin (plugin/mcpctl.ts): headless mount under a stable 'mcpctl'
  name, re-asserted on first contact + before each turn; skips re-registering
  an unchanged mount so a gated project's begin_session state survives.
- TUI plugin (mcpctl/mcpctl-tui.tsx): /mcpctl filterable picker (live switch),
  /mcpctl-status, /mcpctl-skills, and a mcpctl:<project> footer indicator
  right of the model name, directly above the token counter.
- mcpctl config opencode: mint/reuse project mcptoken, install + register
  plugins, write 0600 state, write marker, sync skills.
- skills sync --agent opencode: new shared-tree target (XDG-aware).
- shared credential plumbing lifted from config prime-agent and parameterised
  by agent so both hosts share mint/reuse/retire logic.
- embedded-source generator + freshness test; typechecked against the real
  @opencode-ai/plugin types (1.18.15); unit tests for settings, order, embed.

typecheck (incl. opencode-ext) and the full cli suite (619 tests) pass.

README.md

@@ -195,6 +195,49 @@ Inside pi:
 
 See [docs/pi-extension.md](docs/pi-extension.md) for full details.
 
+### Connect opencode
+
+[opencode](https://opencode.ai) supports MCP natively *and* exposes an API for
+its own MCP registry, so mcpctl mounts the project's gateway through the running
+app — switching projects needs **no restart**.
+
+```bash
+mcpctl config opencode --project monitoring
+```
+
+That mints (or reuses) the project token, writes it to a 0600 state file
+(`~/.mcpctl/opencode-state.json` — the token never lands in a mode-0644
+`opencode.json` people paste into bug reports), installs a server plugin into
+`~/.config/opencode/plugin/mcpctl.ts` and a TUI plugin into
+`~/.config/opencode/mcpctl/`, registers the latter in `tui.json`, and syncs
+skills into `~/.config/opencode/skill/`.
+
+Inside opencode:
+
+- `/mcpctl` — filterable project picker; switches **live, no restart**
+- `/mcpctl-status` — active project, mount state, gateway URL
+- `/mcpctl-skills` — re-sync this project's skills
+- a `mcpctl:<project>` footer indicator right of the model name, above the
+  token counter
+
+Skip individual steps:
+
+```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           # state only, leave plugins alone
+mcpctl config opencode --project monitoring --skip-marker           # don't touch .mcpctl-project here
+mcpctl config opencode --project monitoring --dry-run               # print the plan, write nothing
+```
+
+Re-sync skills later:
+
+```bash
+mcpctl skills sync --agent opencode --project monitoring
+```
+
+See [docs/opencode-extension.md](docs/opencode-extension.md) for full details.
+
 ## Declarative Configuration
 
 Everything can be defined in YAML and applied with `mcpctl apply`: