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.

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 +45,8 @@
   "devDependencies": {
     "@earendil-works/pi-ai": "^0.84.1",
     "@earendil-works/pi-coding-agent": "0.84.1",
+    "@opencode-ai/plugin": "1.18.15",
+    "@opentui/solid": "^0.5.1",
     "@types/node": "^25.3.0",
     "@typescript-eslint/eslint-plugin": "^8.56.0",
     "@typescript-eslint/parser": "^8.56.0",