Repair three stale account-directory spawn fixtures

[ruby-dlee]

main's CI has been red for 16h on the account-directory suite, which aborted at its eighth test. Every firstmate PR inherits that failure, so #26 and #28 are both blocked by a red they did not cause.

Three fixtures had drifted from the direct-account spawn path they model:

• The fake treehouse never answered get --lease, so fm-spawn correctly refused to launch without a worktree. It now reports one on stdout, the way treehouse does.
• The fixture worktree was attached to a branch. A leased Treehouse worktree is handed over at detached HEAD and fm-spawn refuses an attached one, so the fixture could never reach the behavior it was asserting.
• The identity-drift fake detached an already-detached worktree — a no-op — so the post-preparation recheck had nothing to catch. It now moves the worktree onto a branch, which is a real drift.

One assertion also pinned the order in which two correct guards fire rather than the refusal itself. It now accepts either and additionally requires the refusal to name the offending path, so it tests the guarantee instead of the implementation.

Twelve tests go from unreachable to passing. The one remaining failure is the failed-spawn worktree leak itself — a genuine defect, under repair separately, and not something to paper over here.