refactor(#1268): re-home core re-export-spine squatters + migration-convergence lint (#1272)

Re-home the 6 implementation functions squatting in the core.cjs re-export
spine (ADR-857) into the modules whose interface they belong to, with core
re-exporting them BY REFERENCE so all 32 callers + the shim-identity tests
keep resolving unchanged:
- worktree-safety: resolveWorktreeRoot, pruneOrphanedWorktrees
- git-base-branch (broadened to the Git Query Module): gitWorktreeInfoInternal
- agent-install-check (new leaf): getAgentsDir, checkAgentsInstalled
- delete the _resetRuntimeWarningCacheForTests wrapper; consumers use a
  shared resetRuntimeWarningCaches() helper in tests/helpers.cjs

Add scripts/lint-core-spine-imports.cjs (migration-convergence lint with a
30-importer allowlist, wired into lint:ci) so the staged spine retirement
provably converges: CI fails on any new ./core import. Register the new
generated agent-install-check.cjs in eslint-ignore + .gitignore +
INVENTORY-MANIFEST.json.

No behaviour change. First tranche (T0) of epic #1267.

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

src/git-base-branch.cts

@@ -199,6 +199,38 @@ export function resolveBaseBranch(
   return 'main';
 }
 
+// ─── gitWorktreeInfoInternal (moved from core.cjs, ADR-857 T0 #1268) ─────────
+
+export interface GitWorktreeInfo {
+  inside: boolean;
+  worktreeRoot: string | null;
+}
+
+/**
+ * Detect whether `cwd` sits inside a git worktree, and if so, return the
+ * absolute path of the worktree root.
+ */
+export function gitWorktreeInfoInternal(cwd: string): GitWorktreeInfo {
+  try {
+    const insideResult = execGitSeam(['rev-parse', '--is-inside-work-tree'], { cwd, timeout: 5000 });
+    if (insideResult.exitCode !== 0) {
+      return { inside: false, worktreeRoot: null };
+    }
+    const insideStdout = String(insideResult.stdout || '').trim();
+    if (insideStdout !== 'true') {
+      return { inside: false, worktreeRoot: null };
+    }
+    const rootResult = execGitSeam(['rev-parse', '--show-toplevel'], { cwd, timeout: 5000 });
+    if (rootResult.exitCode !== 0) {
+      return { inside: true, worktreeRoot: null };
+    }
+    const root = String(rootResult.stdout || '').trim();
+    return { inside: true, worktreeRoot: root || null };
+  } catch {
+    return { inside: false, worktreeRoot: null };
+  }
+}
+
 // ─── CLI entry point ──────────────────────────────────────────────────────────
 
 /**