feat(opencode): native opencode addon — bottom-row project indicator + in-TUI switcher

Adds an opencode integration mirroring the existing pi and prime-agent addons.

The addon is a TUI plugin (`src/opencode-ext/open-mcpctl.tsx`) that:
- renders the active mcpctl project into opencode's `app_bottom` slot — the
  status row that also carries the token counter / model info,
- registers a `mcpctl.switch_project` command bound to `<leader>m` that opens a
  searchable `DialogSelect` project picker,
- switches live by re-pointing opencode's `mcpctl` MCP server at the new
  project's mcplocal endpoint via the SDK client (native MCP, no JSON-RPC code),
- persists state in `~/.mcpctl/opencode-state.json` (+ `.mcpctl-project` marker).

Pure helpers (`filterProjects`, `toolChangeAnnouncement`) live in
`projects.ts` so they are unit-tested.

`mcpctl config opencode --project X` provisions it: writes the plugin into
`~/.config/opencode/plugin/`, registers the `.tsx` in opencode.json's plugin
array (auto-discovery only matches {ts,js}), best-effort installs the TUI peer
deps (`@opentui/*`), and syncs skills into `~/.config/opencode/skill`.
`mcpctl skills sync --agent opencode` is added as a fourth shared-tree target.

Closes typecheck gap with `typecheck:opencode-ext` against the real
`@opencode-ai/plugin` + `@opentui/solid` types.

Tests: embed freshness, opencode-settings, projects helpers, skills agent
root. Full `pnpm run typecheck` + cli tests pass; completions regenerated.

scripts/generate-opencode-extension.ts

@@ -0,0 +1,50 @@
+#!/usr/bin/env node
+/**
+ * Generates `src/cli/src/config/opencode-extension.ts` which embeds the opencode
+ * plugin source (`open-mcpctl.tsx` + `projects.ts`) as string constants,
+ * mirroring how the pi and prime-agent extensions are embedded.
+ *
+ * Embedding matters: `mcpctl config opencode` must work from an installed
+ * binary that has no access to the source tree. The installed plugin files are
+ * this exact embedded source, so what the CLI ships is always what opencode
+ * runs.
+ *
+ * Regenerate after editing the plugin source:
+ *   npx tsx scripts/generate-opencode-extension.ts
+ */
+import { readFileSync, writeFileSync, mkdirSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+
+const scriptsDir = import.meta.dirname;
+const root = join(scriptsDir, '..');
+const opencodeExtDir = join(root, 'src', 'opencode-ext');
+
+const MAIN = readFileSync(join(opencodeExtDir, 'open-mcpctl.tsx'), 'utf-8');
+const PROJECTS = readFileSync(join(opencodeExtDir, 'projects.ts'), 'utf-8');
+
+function embed(src) {
+  // JSON.stringify yields a quoted string literal we can inline directly.
+  return JSON.stringify(src);
+}
+
+const out = `/**
+ * Embedded source of the mcpctl opencode plugin — DO NOT EDIT BY HAND.
+ * Generated by \`npx tsx scripts/generate-opencode-extension.ts\` from
+ * \`src/opencode-ext/open-mcpctl.tsx\` + \`src/opencode-ext/projects.ts\`.
+ *
+ * \`mcpctl config opencode\` writes these verbatim into
+ * ~/.config/opencode/plugin/ and registers \`open-mcpctl.tsx\` in opencode.json's
+ * plugin array, so an installed binary with no source tree can still provision
+ * a working opencode integration.
+ */
+export const OPENCODE_EXTENSION_FILENAMES = ['open-mcpctl.tsx', 'projects.ts'] as const;
+
+export const OPENCODE_EXTENSION_FILES: Record<string, string> = {
+  'open-mcpctl.tsx': ${embed(MAIN)},
+  'projects.ts': ${embed(PROJECTS)},
+};
+`;
+
+mkdirSync(dirname(join(root, 'src', 'cli', 'src', 'config')), { recursive: true });
+writeFileSync(join(root, 'src', 'cli', 'src', 'config', 'opencode-extension.ts'), out);
+console.log('wrote src/cli/src/config/opencode-extension.ts');