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).

scripts/generate-opencode-extension.ts

@@ -0,0 +1,62 @@
+#!/usr/bin/env node
+/**
+ * Generates `src/cli/src/config/opencode-extension.ts`, which embeds the two
+ * opencode plugin sources as string constants — mirroring how the pi extension
+ * (`pi-extension.ts`) and prime-agent's `/mcpctl` switcher
+ * (`prime-agent-extension.ts`) 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 sources:
+ *   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 extDir = join(root, 'src', 'opencode-ext');
+
+const SERVER = readFileSync(join(extDir, 'mcpctl-opencode.ts'), 'utf-8');
+const TUI = readFileSync(join(extDir, 'mcpctl-opencode-tui.tsx'), 'utf-8');
+
+function embed(src: string): string {
+  // JSON.stringify yields a quoted string literal we can inline directly.
+  return JSON.stringify(src);
+}
+
+const out = `/**
+ * Embedded source of the mcpctl opencode plugins — DO NOT EDIT BY HAND.
+ * Generated by \`npx tsx scripts/generate-opencode-extension.ts\` from
+ * \`src/opencode-ext/mcpctl-opencode.ts\` and
+ * \`src/opencode-ext/mcpctl-opencode-tui.tsx\`.
+ *
+ * \`mcpctl config opencode\` writes these verbatim into ~/.config/opencode/
+ * (and registers them), so an installed binary with no source tree can still
+ * provision a working opencode integration.
+ *
+ * The install file names differ from the source file names on purpose:
+ *   - the server plugin lands in opencode's auto-discovered \`plugin/\` dir,
+ *     where its basename is what shows up in \`opencode plugin list\`;
+ *   - the TUI plugin lands beside it in \`mcpctl/\`, referenced by path from
+ *     \`tui.json\`, and must keep a \`.tsx\` extension for opencode to transpile
+ *     its JSX.
+ */
+
+/** Install name of the server plugin, relative to ~/.config/opencode/plugin/. */
+export const OPENCODE_SERVER_PLUGIN_FILENAME = 'mcpctl.ts';
+
+/** Install name of the TUI plugin, relative to ~/.config/opencode/mcpctl/. */
+export const OPENCODE_TUI_PLUGIN_FILENAME = 'mcpctl-tui.tsx';
+
+export const OPENCODE_SERVER_PLUGIN_SOURCE: string = ${embed(SERVER)};
+
+export const OPENCODE_TUI_PLUGIN_SOURCE: string = ${embed(TUI)};
+`;
+
+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');