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)

.changeset/humble-geese-roam.md

@@ -0,0 +1,7 @@
+---
+type: Changed
+pr: 1804
+---
+**Internal: the model adapter seam exposes `passive` + `active` adapters selected by `modelMode`** — `createModelAdapter({modelMode})` (new `src/model-adapter.cts`): `passive` formalizes today's tier routing (delegates to `model-resolver.resolveModelForTier`), `active` is a host-supplied `sendRequest` seam (VS Code `vscode.lm` / pi providers), fail-closed until Phase 5 binds a concrete provider (ADR-1239 Phase C-1 / #1680 AC3). No user-facing change — the seam is not yet wired to any runtime path.
+
+<!-- docs-exempt: internal adapter seam; no user-facing command/flag/config/schema/doc surface -->