chore(#57): add runtime install no-drift guard tests (#867)

Add tests/issue-57-runtime-install-no-drift.test.cjs protecting the Runtime
Install Policy Module boundary (ADR-58) and the explicit Runtime Config Adapter
Registry (#60), now that #58/#60/#56 have landed and the seam exists.

The guards fail when:
- (AC1) supported-runtime metadata is added to an installer/query call site
  (allRuntimes, the interactive runtimeMap menu) without a matching registry
  adapter entry — enforced by three-way set equality across allRuntimes,
  runtimeMap values, and ALLOWED_CONFIG_RUNTIMES.
- (AC2) config-mutation dispatch escapes the registry: every intent uses a
  registry-declared install surface, every permission writer is null or a
  registry-known runtime, unknown/prototype-key runtimes fail loudly, and a new
  inline 'runtime === "..."' branch against an unregistered runtime is rejected.

Assertions are behavioral (require + reflect on live exports) where behavior can
cover the contract (AC3); two annotated structural guards cover what it cannot.
Existing installer/runtime-policy/runtime-global-skills suites stay green (AC4).

Gates: eslint, lint-test-file-count, full Mac suite (12715 pass / 0 fail) and
Linux Docker (14709 pass / 0 fail) all green; Codex adversarial-review,
/code-review, and /security-review run with findings addressed.

Closes #57

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
(cherry picked from commit 35174ce9b0)

.changeset/runtime-install-no-drift-tests.md

@@ -0,0 +1,6 @@
+---
+type: Changed
+pr: 867
+---
+Added no-drift guard tests (`tests/issue-57-runtime-install-no-drift.test.cjs`) that protect the Runtime Install Policy Module boundary (ADR-58) and the explicit Runtime Config Adapter Registry (#60). They fail loudly when supported-runtime metadata is added to an installer call site (`allRuntimes`, the interactive `runtimeMap` menu) without a matching registry adapter entry, or when config-mutation dispatch escapes the registry's declared install surfaces — catching reintroduction of the scattered per-runtime branching those seams removed.
+<!-- docs-exempt: test-only — adds architecture-guard tests, no user-facing behavior, command, or config change -->