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)

src/model-adapter.cts

@@ -0,0 +1,78 @@
+/**
+ * Model adapter seam (ADR-1239 Phase C-1, AC3 / #1680).
+ *
+ * Two model-layer adapters selected by the negotiated `modelMode` axis
+ * (host-integration.cts):
+ *
+ *   - `passive` — GSD can only inject prompts / a per-agent `model` field (the
+ *     CLI runtimes: claude/gemini/codex/opencode/cursor/…). Formalizes today's
+ *     tier routing from src/model-resolver.cts: `resolveModel` delegates
+ *     straight to `resolveModelForTier`, so passive reproduces current behavior
+ *     byte-for-behavior.
+ *   - `active` — the host exposes a provider `sendRequest` (VS Code `vscode.lm`,
+ *     pi providers). GSD calls the model through the host. Ships here as a SEAM:
+ *     a host-supplied `sendRequest` slot, fail-closed until a real consumer
+ *     binds it (Phase 5 / #1682).
+ *
+ * Minimal (per ADR-1239 open wire-shape question): one factory, two shapes
+ * discriminated by `mode`. Concrete provider protocol (request/response shape)
+ * is fixed when a real active host lands in Phase 5.
+ */
+'use strict';
+
+// eslint-disable-next-line @typescript-eslint/no-require-imports
+import modelResolver = require('./model-resolver.cjs');
+
+export type ModelMode = 'passive' | 'active';
+
+export interface ModelAdapter {
+  readonly mode: ModelMode;
+}
+
+export interface PassiveModelAdapter extends ModelAdapter {
+  readonly mode: 'passive';
+  /** Resolve a model id for a tier. Delegates to model-resolver's tier routing. */
+  resolveModel(args: { cwd: string; agentType: string; attempt?: number }): string;
+}
+
+export interface ActiveModelAdapter extends ModelAdapter {
+  readonly mode: 'active';
+  /** Host-supplied model-call primitive. Throws (fail-closed) if not bound. */
+  sendRequest(req: unknown): unknown;
+}
+
+export interface CreateModelAdapterOptions {
+  /** Required for `active`: the host's model-call primitive. Ignored for `passive`. */
+  sendRequest?: (req: unknown) => unknown;
+}
+
+export function createModelAdapter(
+  { modelMode }: { modelMode: ModelMode },
+  options: CreateModelAdapterOptions = {},
+): ModelAdapter {
+  if (modelMode !== 'passive' && modelMode !== 'active') {
+    throw new TypeError(`createModelAdapter: modelMode must be 'passive' | 'active' (got ${JSON.stringify(modelMode)})`);
+  }
+  if (modelMode === 'passive') {
+    return Object.freeze({
+      mode: 'passive' as const,
+      resolveModel({ cwd, agentType, attempt }: { cwd: string; agentType: string; attempt?: number }): string {
+        return modelResolver.resolveModelForTier(cwd, agentType, attempt);
+      },
+    });
+  }
+  // active: bind the host's sendRequest, fail-closed if absent.
+  const sendRequest = options.sendRequest;
+  return Object.freeze({
+    mode: 'active' as const,
+    sendRequest(req: unknown): unknown {
+      if (typeof sendRequest !== 'function') {
+        throw new Error(
+          'ActiveModelAdapter.sendRequest: no host provider bound — the active model seam ' +
+          'requires a sendRequest primitive from the host (Phase 5 wires a concrete provider).',
+        );
+      }
+      return sendRequest(req);
+    },
+  });
+}