memory writer creates corrupt directory trees from unvalidated path segments (absolute paths, 0, null)

[agents-squads]

The memory-write path builds directories from unvalidated variables. Observed corruption classes in real workspaces:

1. An absolute file path used as a subdirectory chain: .agents/memory/<squad>/Users/<user>/.../<agent>.md/executions.md (a .md FILE name becoming a directory).
2. Falsy values as directory names: .agents/memory/0/, .agents/memory/null/ — created as recently as today.

Fix: validate path segments before mkdir/write — reject absolute-path fragments, empty/falsy stringifications (0, null, undefined, None), and any segment ending in .md; fail loud instead of writing. Add a unit test per corruption class.

15+ corrupt trees found in a corpus audit; they will be purged, but the writer must stop producing them or the cleanup is Sisyphean.