fix: configurable node-local verifier cache

[samsja]

Support VERIFIERS_CACHE_DIR (including ~ expansion) so large local eval runs can keep verifier cache lookups off shared NFS. The default remains ~/.cache/verifiers; an empty override retains the default.

Validation: Ruff and direct default/absolute-path/tilde-override assertions pass. This is a cache-root selection change, not a concurrency limiter or an NFS filesystem fix.

Note

Add VERIFIERS_CACHE_DIR environment variable override for verifier cache path

Adds a new cache_dir resolver in paths.py that expands VERIFIERS_CACHE_DIR when set, otherwise falls back to the existing temporary-home-based default. The module-level CACHE_DIR now initializes through this resolver at import time.

^{Macroscope summarized d4f446e.}