When slinging to an existing polecat, the bd update command was running
from town root which doesn't support prefix-based routing for writes.
Fix: Capture the target agent's working directory from resolveTargetAgent
and use it as hookWorkDir. This ensures bd update runs from the polecat's
worktree where the .beads/redirect file enables routing to the correct
database.
Also fixed the self-sling case to capture and use selfWorkDir.
58e7c96936