Pull the disk-cache + FetchResult substrate out of bitwarden.py into a new
agent/secret_sources/_cache.py: FetchResult, CachedFetch, is_valid_env_name,
and a generic DiskCache (atomic mkstemp -> chmod 0600 -> os.replace write,
0700 cache dir, TTL-gated read AND write). Bitwarden now consumes it via a
module-level DiskCache instance and thin wrappers, so the security-sensitive
atomic-write/0600/TTL logic lives in exactly one place instead of being
copy-pasted per backend (and drifting). Behavior is unchanged — the full
Bitwarden suite passes untouched.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
