Model cache fast-follows: sharedfs data-sharing probe, Samba infra reconciliation, ephemeral env filtering

Follow-ups deferred from #227 (also listed in the SDD Known gaps section):

1. sharedfs reader binding assumes the StorageClass exposes one shared filesystem. Separately provisioned PVCs do not share data on provisioners that create per-claim access points, subvolumes, or directories (EFS access points, CephFS subvolumes, some NFS provisioners). The capability probe validates bindability, not data-sharing. Fast-follow: a write-through-one-claim / read-through-another probe, and deriving reader PVs from the writer's bound volume where the driver supports it.

2. The per-handle Samba infrastructure (EnsureSambaModelCacheInfra) is create-once: image, resource, and cache-size changes do not reconcile onto an existing server, and the backing PVC is never expanded for a larger cache-size request. Fast-follow: reconcile mutable Deployment fields and expand the PVC, keeping credentials create-once.

3. The ephemeral model-cache-init env forwards the full launch environment through the miniservice metadata ConfigMap. Fast-follow: narrow it to the variables the init container consumes and source sensitive values from Secrets.

Relates to #226