feat(#1680): ADR-1239 Phase C-1 — model adapter seam (passive + active) [AC3] (#1804)

* feat(#1680): ADR-1239 Phase C-1 — model adapter seam (passive + active) [AC3]

Phase 3 slice 3 (AC3). Two model-layer adapters selected by the negotiated
modelMode axis (host-integration.cts):

- passive: formalizes today's tier routing from src/model-resolver.cts —
  resolveModel delegates straight to resolveModelForTier (byte-for-behavior).
  This is the CLI runtimes (claude/gemini/codex/opencode/cursor/...): GSD injects
  prompts / a per-agent model field.
- active: a host-supplied sendRequest seam (VS Code vscode.lm / pi providers) —
  GSD calls the model through the host. Ships as a fail-closed seam (throws until
  a provider is bound); Phase 5 wires a concrete provider.

createModelAdapter({modelMode}, {sendRequest?}) — factory gating throws on
invalid mode. Proactive CI gates applied: ADR-457 eslint ignores entry +
INVENTORY-MANIFEST cli_modules entry + comment-wording audited for the
injection-scan substring trap.

* chore(changeset): add Changed fragment for model adapter seam (#1680)

tests/model-adapter.test.cjs

@@ -0,0 +1,73 @@
+'use strict';
+/**
+ * Tests for the model adapter seam (ADR-1239 Phase C-1, AC3 / #1680).
+ *
+ * Pins:
+ *   1. PASSIVE — `resolveModel` delegates to model-resolver's tier routing
+ *      (reproduces today's behavior).
+ *   2. ACTIVE — `sendRequest` calls the host-supplied primitive; FAIL-CLOSED
+ *      (throws) when no provider is bound.
+ *   3. FACTORY GATING — invalid modelMode throws.
+ *
+ * Behavioral tests only; delegation verified via module-ref monkeypatch.
+ */
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const { createModelAdapter } = require('../gsd-core/bin/lib/model-adapter.cjs');
+const modelResolver = require('../gsd-core/bin/lib/model-resolver.cjs');
+
+test('passive model adapter: resolveModel delegates to model-resolver tier routing (reproduces today behavior)', () => {
+  const adapter = createModelAdapter({ modelMode: 'passive' });
+  assert.strictEqual(adapter.mode, 'passive');
+  const original = modelResolver.resolveModelForTier;
+  let captured = null;
+  modelResolver.resolveModelForTier = function (...a) { captured = a; return 'sonnet'; };
+  try {
+    const out = adapter.resolveModel({ cwd: '/tmp/proj', agentType: 'planner', attempt: 2 });
+    assert.strictEqual(out, 'sonnet', 'resolveModel must return the resolver result');
+    assert.deepStrictEqual(captured, ['/tmp/proj', 'planner', 2], 'must delegate (cwd, agentType, attempt) verbatim');
+  } finally {
+    modelResolver.resolveModelForTier = original;
+  }
+});
+
+test('passive model adapter: attempt is optional (delegates undefined when omitted)', () => {
+  const adapter = createModelAdapter({ modelMode: 'passive' });
+  const original = modelResolver.resolveModelForTier;
+  let captured = null;
+  modelResolver.resolveModelForTier = function (...a) { captured = a; return 'haiku'; };
+  try {
+    adapter.resolveModel({ cwd: '/tmp/proj', agentType: 'executor' });
+    assert.deepStrictEqual(captured, ['/tmp/proj', 'executor', undefined], 'attempt defaults to undefined');
+  } finally {
+    modelResolver.resolveModelForTier = original;
+  }
+});
+
+test('active model adapter: sendRequest invokes the bound host provider', () => {
+  const calls = [];
+  const adapter = createModelAdapter(
+    { modelMode: 'active' },
+    { sendRequest: (req) => { calls.push(req); return { ok: true, echo: req }; } },
+  );
+  assert.strictEqual(adapter.mode, 'active');
+  const out = adapter.sendRequest({ prompt: 'hi' });
+  assert.deepStrictEqual(calls, [{ prompt: 'hi' }], 'host provider invoked with the request');
+  assert.deepStrictEqual(out, { ok: true, echo: { prompt: 'hi' } }, 'host provider return value passed through');
+});
+
+test('active model adapter: FAIL-CLOSED when no host provider is bound (sendRequest throws)', () => {
+  const adapter = createModelAdapter({ modelMode: 'active' });
+  assert.throws(
+    () => adapter.sendRequest({ prompt: 'hi' }),
+    /no host provider bound/,
+    'sendRequest must throw when no provider is bound (fail-closed, never silently no-op)',
+  );
+});
+
+test('createModelAdapter: invalid modelMode throws (fail-closed construction)', () => {
+  for (const bad of ['host', 'dynamic', '', null, undefined, 3]) {
+    assert.throws(() => createModelAdapter({ modelMode: bad }), TypeError, `modelMode=${JSON.stringify(bad)} must throw TypeError`);
+  }
+});