feat(#1682): OpenCode session.idle + opencode-subset dialect + Claude parity — Slice 1b/c (#1930)

* feat(#1682): OpenCode session.idle + opencode-subset dialect + Claude parity — Slice 1b/c

- Plugin (.opencode/plugins/gsd-core.js): handle session.idle (↔ Claude Stop
  lifecycle point; no-op sentinel — state already persisted to .planning/).
  Completes the compaction/idle pair (#1914 shipped compaction).
- Declare hookEvents: 'opencode-subset' in the OpenCode descriptor — the
  reserved dialect now has a real consumer (no longer zero-consumer).
- host-integration.cts: add HOOK_EVENT_SURFACES + hookEventSurfaceFor() — the
  pure consumer that resolves a dialect to its host-fireable event surface.
  opencode-subset = session/tool/file subset with NO workflow-phase events
  (engine owns phase sequencing; ADR-1239 §OpenCode binding).
- Tests: hookEventSurfaceFor unit tests (claude/gemini/opencode-subset/null);
  plugin session.idle no-throw; compaction breadcrumb; opencode-subset surface
  parity vs the plugin's handlers (Claude parity).

* fix(#1682): don't declare hookEvents on opencode (hooksSurface:none invariant)

The repo invariant couples runtime.hookEvents to the managed settings.json hook
surface: hooksSurface:'none' runtimes (opencode — plugin owns hooks) must NOT
declare hookEvents. Declaring 'opencode-subset' there violated 3 capability-
registry invariants + 2 install-plan golden masters + opencode golden parity.

The opencode-subset dialect is still IMPLEMENTED — just not via the legacy
descriptor field: hookEventSurfaceFor() (host-integration.cts) is its consumer,
and the OpenCode plugin consumes the subset events at runtime (session.idle
added here; compaction shipped in #1914). Declaring it on the descriptor would
require weakening the hooksSurface:none ⇔ no-hookEvents invariant (flagged for
decision).

* fix(#1682): refresh opencode golden parity for plugins/gsd-core.js (session.idle)

* docs(changeset): OpenCode session.idle + opencode-subset dialect (#1682)

* docs(changeset): backfill PR #1930

tests/opencode-plugin-adapter.test.cjs

@@ -300,6 +300,54 @@ test('config hook is a no-op in installed (non-package) layout', async (t) => {
   assert.deepEqual(config, {});
 });
 
+// ---------------------------------------------------------------------------
+// Session lifecycle + opencode-subset surface parity (#1682 Slice 1b/c)
+// ---------------------------------------------------------------------------
+
+test('session.idle event is handled (no-op sentinel) without throwing', async (t) => {
+  const { mod } = buildInstalledLayout(t, {});
+  const handlers = await mod.server({ directory: process.cwd() });
+  // session.idle ↔ Claude Stop lifecycle point; recognized no-op today.
+  await assert.doesNotReject(() => handlers.event({ event: { type: 'session.idle' } }));
+});
+
+test('experimental.session.compacting injects the GSD state breadcrumb', async (t) => {
+  const { mod } = buildInstalledLayout(t, { 'gsd-context-monitor.js': stubHook('') });
+  const handlers = await mod.server({ directory: process.cwd() });
+  // Compaction fires only with an active session; session.created sets it.
+  await handlers.event({
+    event: { type: 'session.created', properties: { info: { id: 's1', directory: process.cwd() } } },
+  });
+  const output = {};
+  await handlers['experimental.session.compacting']({}, output);
+  assert.ok(Array.isArray(output.context) && output.context.length > 0, 'compaction injects a GSD breadcrumb');
+  assert.ok(output.context.some((c) => /GSD/.test(c)), 'breadcrumb is GSD-tagged');
+});
+
+test('plugin implements the full declared opencode-subset hook surface (Claude parity)', async (t) => {
+  const { hookEventSurfaceFor } = require('../gsd-core/bin/lib/host-integration.cjs');
+  const surface = hookEventSurfaceFor('opencode-subset');
+  assert.ok(surface, 'opencode-subset is a consumed dialect (non-null surface)');
+  // The engine — not the host bus — owns workflow-phase sequencing on this host.
+  assert.ok(!surface.some((e) => /plan:|verify:|ship:|execute:/.test(e)),
+    'opencode-subset fires no workflow-phase events');
+
+  const { mod } = buildInstalledLayout(t, {});
+  const handlers = await mod.server({ directory: process.cwd() });
+  // Tool + compaction events are top-level handler keys.
+  for (const ev of ['tool.execute.before', 'tool.execute.after', 'experimental.session.compacting']) {
+    assert.equal(typeof handlers[ev], 'function', `plugin exposes a handler for ${ev}`);
+  }
+  // Session/file events dispatch through the `event` handler.
+  assert.equal(typeof handlers.event, 'function', 'plugin exposes an event dispatcher');
+  // Every declared surface event resolves to a plugin handler.
+  for (const ev of surface) {
+    const covered = typeof handlers[ev] === 'function'
+      || ev === 'session.created' || ev === 'session.idle' || ev === 'file.edited';
+    assert.ok(covered, `plugin covers opencode-subset event: ${ev}`);
+  }
+});
+
 // ---------------------------------------------------------------------------
 // Installer integration: copy → manifest → uninstall (real bin/install.js)
 // ---------------------------------------------------------------------------