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).

package.json

@@ -16,7 +16,7 @@
     "clean": "pnpm -r run clean && rimraf node_modules",
     "db:up": "docker compose -f deploy/docker-compose.yml up -d",
     "db:down": "docker compose -f deploy/docker-compose.yml down",
-    "typecheck": "tsc --build && pnpm run typecheck:pi-ext",
+    "typecheck": "tsc --build && pnpm run typecheck:pi-ext && pnpm run typecheck:opencode-ext",
     "completions:generate": "tsx scripts/generate-completions.ts --write",
     "completions:check": "tsx scripts/generate-completions.ts --check",
     "rpm:build": "bash scripts/build-rpm.sh",
@@ -34,6 +34,7 @@
     "mcpd:deploy-dry": "bash deploy.sh --dry-run",
     "mcpd:logs": "bash logs.sh",
     "typecheck:pi-ext": "tsc -p src/pi-ext/tsconfig.json",
+    "typecheck:opencode-ext": "tsc -p src/opencode-ext/tsconfig.json",
     "smoke:clean": "tsx scripts/clean-smoke-resources.ts"
   },
   "engines": {
@@ -44,6 +45,10 @@
   "devDependencies": {
     "@earendil-works/pi-ai": "^0.84.1",
     "@earendil-works/pi-coding-agent": "0.84.1",
+    "@opencode-ai/plugin": "1.18.15",
+    "@opentui/core": "0.5.1",
+    "@opentui/keymap": "0.5.1",
+    "@opentui/solid": "0.5.1",
     "@types/node": "^25.3.0",
     "@typescript-eslint/eslint-plugin": "^8.56.0",
     "@typescript-eslint/parser": "^8.56.0",
@@ -51,6 +56,7 @@
     "eslint": "^10.0.1",
     "eslint-config-prettier": "^10.1.8",
     "rimraf": "^6.1.3",
+    "solid-js": "1.9.12",
     "tsx": "^4.21.0",
     "typebox": "1.3.11",
     "typescript": "^5.9.3",