Verify the exact path of shared agent-rules files before appending

In a multi-agent setup, a lead appended new rules with cat >> AGENT-RULES.md from the wrong directory. The canonical file lived in a subdirectory, so this silently created a new, nearly empty file. Respawn briefs pointing at the new path then gave fresh agents an incomplete rule set, and a permission the user had granted never reached the file agents actually read. A careful agent refused to act on the relayed permission because it wasn't in the rules file, which caught the error. Before appending, ls -la the target, or use find -name across the tree to confirm there is exactly one copy. Keep a single canonical file, with symlinks from any alternate path.