test(worktree): take the trusted PATH fixture from os.defpath, not the host

The full gate runs in a Nix-based container where true resolves into /nix/store, which is group-writable and therefore never root-controlled, so the two trusted_path tests failed there. They now take the first root-controlled os.defpath directory and skip when there is none. A sticky group-writable ancestor stays refused: on multi-user Nix any user can have the daemon build root-owned store content (skeptic round 7); the README says to pin an absolute path on Nix.

🎯 Fix: 92 %
💥 Break: 1 %
💸 Cost: 100 %
⏱️ Time: 100 %
🛡️ Security: 100 %
🏆 Score: 91
🚦 Gate: ✅ 90/5, cost and time <= 100, security 100 %

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -90,7 +90,9 @@ replaced; a
 reported, never run, because an agent may have written it. The pinned program
 must be an absolute path or a root-owned command on a `PATH` entry that root
 owns and no group or other user can write, up to `/`, so a binary planted in a
-user-writable directory such as a virtualenv never runs. A bare name runs as
+user-writable directory such as a virtualenv never runs. The Nix store never
+counts: it is group-writable, and any user can have the daemon build
+root-owned content there, so pin an absolute path on Nix. A bare name runs as
 the resolved file that was checked. Inside a user namespace other than the
 initial one, root means the overflow uid, under which the real root's files
 show there, as do those of every other unmapped owner. Outside such a