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.

scripts/generate-opencode-extension.ts

@@ -0,0 +1,55 @@
+#!/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/
+ * so an installed binary with no source tree can still provision a working
+ * opencode integration.
+ */
+
+/** 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');