feat: add OpenCode adapter

Thin OpenCode plugin injecting the ponytail ruleset via experimental.chat.system.transform, reusing the shared instruction builder. Verified end-to-end on OpenCode 1.17.4. Supersedes #15.

tests/opencode-plugin.test.js

@@ -0,0 +1,64 @@
+#!/usr/bin/env node
+// Smoke test for the OpenCode adapter: the plugin's hooks behave against the
+// real (structural) OpenCode hook shapes. No live OpenCode needed.
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('fs');
+const os = require('os');
+const path = require('path');
+const { pathToFileURL } = require('url');
+
+// Point the plugin's mode-flag at a temp config home BEFORE it loads — the
+// plugin resolves its state path once at load (as it does under a real OpenCode
+// process, where XDG_CONFIG_HOME is already set). The dynamic import below runs
+// after this assignment, so the ordering holds.
+const tmp = fs.mkdtempSync(path.join(os.tmpdir(), 'ponytail-opencode-'));
+process.env.XDG_CONFIG_HOME = tmp;
+delete process.env.PONYTAIL_DEFAULT_MODE;
+const statePath = path.join(tmp, 'opencode', '.ponytail-active');
+
+let loadPlugin;
+test.before(async () => {
+  const url = pathToFileURL(path.join(__dirname, '..', '.opencode', 'plugins', 'ponytail.mjs'));
+  loadPlugin = (await import(url)).default;
+});
+
+function transform(hooks) {
+  const output = { system: [] };
+  return hooks['experimental.chat.system.transform']({ model: {} }, output).then(() => output.system);
+}
+
+test('system.transform injects the ruleset at the default mode (full)', async () => {
+  try { fs.unlinkSync(statePath); } catch (e) {}
+  const hooks = await loadPlugin({});
+  const system = await transform(hooks);
+  assert.equal(system.length, 1);
+  assert.match(system[0], /PONYTAIL MODE ACTIVE — level: full/);
+  assert.match(system[0], /lazy senior developer/);
+});
+
+test('command.execute.before persists /ponytail ultra, transform follows it', async () => {
+  const hooks = await loadPlugin({});
+  await hooks['command.execute.before']({ command: 'ponytail', arguments: 'ultra', sessionID: 's' });
+  assert.equal(fs.readFileSync(statePath, 'utf8'), 'ultra');
+  const system = await transform(hooks);
+  assert.match(system[0], /PONYTAIL MODE ACTIVE — level: ultra/);
+});
+
+test('/ponytail off persists off and transform injects nothing', async () => {
+  const hooks = await loadPlugin({});
+  await hooks['command.execute.before']({ command: 'ponytail', arguments: 'off', sessionID: 's' });
+  assert.equal(fs.readFileSync(statePath, 'utf8'), 'off');
+  const system = await transform(hooks);
+  assert.deepEqual(system, []);
+});
+
+test('unrelated commands do not touch the flag', async () => {
+  try { fs.unlinkSync(statePath); } catch (e) {}
+  const hooks = await loadPlugin({});
+  await hooks['command.execute.before']({ command: 'commit', arguments: 'x', sessionID: 's' });
+  assert.equal(fs.existsSync(statePath), false);
+});
+
+test.after(() => fs.rmSync(tmp, { recursive: true, force: true }));