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).

docs/opencode-extension.md

@@ -0,0 +1,116 @@
+# mcpctl × opencode — native MCP integration
+
+## Motivation
+
+opencode is a native MCP client, so mcpctl can mount a project's MCP servers
+without any bridge process. `mcpctl config opencode --project X`:
+
+1. installs two plugins into opencode's config directory, and
+2. provisions the bearer credential the gateway needs.
+
+Switching projects stays **live** — no restarting opencode — which is the
+difference between this and a static `mcp` block in `opencode.json`.
+
+## How it works
+
+Both plugins read `~/.mcpctl/opencode-state.json` (0600):
+
+```jsonc
+{
+  "project": "monitoring",
+  "gatewayUrl": "https://mcp.ad.itaz.eu",
+  "tokens": {
+    "monitoring": "mcpctl_pat_..."
+  }
+}
+```
+
+`opencode.json` is **never touched**. The gateway needs an
+`Authorization: Bearer <mcpctl PAT>` header — a secret that does not belong in a
+mode-0644 config file users paste into bug reports — and the mount has to be
+re-pointable at runtime, which a config file cannot do.
+
+### Server plugin (`plugin/mcpctl.ts`)
+
+Auto-discovered by opencode; covers **headless** runs (`opencode run ...`),
+which load no TUI plugins. It mounts the active project as a remote MCP server
+under the constant name `mcpctl` via opencode's own live MCP API:
+
+- on first server contact (`event`), and
+- before every user turn (`chat.message`), re-reading the state file so a
+  switch made externally takes effect on the next message.
+
+The mount is only re-registered when the `(url, token)` target actually changed
+— re-adding an unchanged config would rebuild the connection and drop mcplocal's
+gated `begin_session` state mid-conversation. Never throws: an unreachable
+gateway degrades to "no mcpctl tools", never to "opencode fails to start".
+
+### TUI plugin (`mcpctl/mcpctl-tui.tsx`, registered in `tui.json`)
+
+Adds to the opencode TUI:
+
+- **`/mcpctl`** — switch the active project from a filterable picker (active
+  project first; terms match name **or** description; a pre-filter prompt is
+  offered when the list is long), matching the prime-agent switcher UX;
+- **`/mcpctl-status`** — show the active project and its MCP mount state;
+- **`/mcpctl-skills`** — re-sync this project's skills into opencode's skill dir;
+- a **`mcpctl:<project>`** indicator in the prompt footer, immediately right of
+  the model name and just above the token counter.
+
+The switch is delegated to the `mcpctl` CLI (`--skip-plugin --skip-marker`), so
+token minting, state and skills stay in the CLI; once the state file is updated
+the TUI re-points the live MCP mount through `api.client.mcp.add`.
+
+## Deliverables
+
+| Artifact | Purpose |
+|----------|---------|
+| `src/opencode-ext/mcpctl-opencode.ts` | Server plugin (headless mount) |
+| `src/opencode-ext/mcpctl-opencode-tui.tsx` | TUI plugin (`/mcpctl` switcher + footer indicator) |
+| `src/cli/src/utils/opencode-settings.ts` | CLI wiring: paths, `tui.json` registration, state file |
+| `mcpctl config opencode` | Provision token + install/register plugins + skill sync |
+| `docs/opencode-extension.md` | This document |
+
+## Skills
+
+`mcpctl config opencode` (when given a project and an API client) syncs the
+project's skills into `~/.config/opencode/skill/`. Re-sync later with:
+
+```bash
+mcpctl skills sync --agent opencode --project <name>
+```
+
+`opencode` is a shared-tree target (like `prime-agent`/`pi`): its own flat skill
+tree and its own state file (`~/.mcpctl/skills-state-opencode.json`), no Claude
+integration.
+
+## Layout & typechecking
+
+The plugins ship as *source*: they are embedded into the CLI
+(`src/cli/src/config/opencode-extension.ts`, generated by
+`scripts/generate-opencode-extension.ts`) and written verbatim into
+`~/.config/opencode/`. The CLI's own build never compiles them, so
+`src/opencode-ext/tsconfig.json` checks them against the **real**
+`@opencode-ai/plugin` types (dev dependency pinned to the opencode release they
+target) — the same guarantee `src/pi-ext/` gives.
+
+```bash
+pnpm run typecheck        # includes typecheck:opencode-ext
+```
+
+After editing `src/opencode-ext/*`, regenerate the embedded copy or the CLI will
+keep installing old sources:
+
+```bash
+npx tsx scripts/generate-opencode-extension.ts
+```
+
+A test (`tests/config/opencode-extension-embed.test.ts`) fails if you forget.
+
+## Authentication caveat
+
+The server and TUI plugins send the stored `mcpctl_pat_` project token as the
+bearer. A locally running `mcpctl-local` daemon does not authenticate
+`/projects/*`, so the header is simply ignored; an authenticated
+`mcplocal serve` accepts only `mcpctl_pat_` bearers (`mcpctl config opencode`
+mints one per project).