refactor(#1291): T6 — migrate test files off the core spine ahead of deletion (#1293)

The convergence lint only scanned src/ + gsd-core/bin, so ~35 test files
still imported core.cjs. Repoint all 33 behaviour importers to the leaf
modules directly (same symbol->leaf map as the src migration; leaves are the
objects core re-exported by reference), delete the now-meaningless
shim-identity describe blocks in the 8 leaf tests, and delete tests/core.test.cjs
(forwarded-behaviour coverage now lives at the leaves; resolveWorktreeRoot
test relocated to worktree-safety in T0) and tests/lint-core-spine-imports.test.cjs
(the lint is removed in T-final). Dropped the stale core.test.cjs entries from
the allow-test-rule-refs allowlist; eslint-rules RuleTester fixture path
pointed at io.cjs.

After T6: ZERO test imports core.cjs. core.cts still builds (now fully unused);
T-final deletes it. No behaviour change.

Closes #1291

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/issue-2517-runtime-aware-profiles.test.cjs

@@ -36,9 +36,11 @@ const {
   resolveModelInternal,
   resolveEffortInternal,
   resolveTierEntry,
+} = require('../gsd-core/bin/lib/model-resolver.cjs');
+const {
   RUNTIME_PROFILE_MAP,
   KNOWN_RUNTIMES,
-} = require('../gsd-core/bin/lib/core.cjs');
+} = require('../gsd-core/bin/lib/model-catalog.cjs');
 const { renderEffortForRuntime } = require('../gsd-core/bin/lib/model-catalog.cjs');
 const { isValidConfigKey } = require('../gsd-core/bin/lib/config-schema.cjs');
 
@@ -471,7 +473,7 @@ describe('issue #2517: VALID_CONFIG_KEYS schema', () => {
 
 // ─── loadConfig validation warnings (review findings #10, #13) ──────────────
 describe('issue #2517: loadConfig warns on unknown runtime/tier (findings #10, #13)', () => {
-  const { loadConfig } = require('../gsd-core/bin/lib/core.cjs');
+  const { loadConfig } = require('../gsd-core/bin/lib/config-loader.cjs');
   let tmpDir;
   let origWrite;
   let captured;