core-skills: cache snapshots before root discovery

[anp-oai]

intent

Config-aware skill snapshots were cached only after rediscovering repo skill roots. On remote filesystems, that repeated metadata probes during session initialization and every turn even when the snapshot itself was reused. Align root discovery with the snapshot cache lifecycle.

implementation

• Key the config snapshot cache from skill-relevant inputs before filesystem I/O.
• Store the filesystem identity with each cache entry so snapshots do not cross filesystem handles.
• Return cache hits before remote root discovery and keep SkillsService::clear_cache() as the shared invalidation boundary.
• Cover an absent repo skill root becoming visible after cache invalidation.

manual validation

• just test -p codex-core-skills skills_for_config_keeps_repo_root_discovery_with_snapshot_cache_lifecycle