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/defaults-json-fallback.test.cjs

@@ -13,7 +13,7 @@ const path = require('path');
 const os = require('os');
 const { cleanup } = require('./helpers.cjs');
 
-const { loadConfig } = require('../gsd-core/bin/lib/core.cjs');
+const { loadConfig } = require('../gsd-core/bin/lib/config-loader.cjs');
 
 /** Create a bare temp dir (no .planning/) to simulate pre-project context */
 function createBareTmpDir() {