chore: tighten local-path-leak guard scope and tooling

- Narrow the worktree-id/T3-path check to docs/superpowers/specs and
  docs/superpowers/plans. Scanning the whole repo flagged any
  legitimate reference to .t3/worktrees or t3code-<hex> (fixtures,
  example configs) and would block unrelated commits.
- Switch the pre-commit hook shebang to bash and call the check via
  an absolute path derived from git rev-parse so the hook works
  regardless of the cwd git happens to invoke it from.
- install-hooks now warns when it overwrites an existing
  core.hooksPath rather than silently clobbering a custom setup.
- Add make verify-local-paths to the AGENTS.md pre-MR checklist so
  contributors run it explicitly even when --no-verify is used.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/check-local-path-leaks.sh

@@ -51,7 +51,7 @@ check_pattern() {
 check_pattern \
 	"T3 worktree path or generated worktree id" \
 	"(${t3_worktree_dir}|/Users/[^[:space:]]*/${t3_worktree_dir}/|${t3_worktree_id_prefix}[0-9a-f]{8})" \
-	.
+	docs/superpowers/specs docs/superpowers/plans
 
 check_pattern \
 	"absolute local filesystem path in generated superpowers docs" \