pimdir backend auto-sources its writes

The pimdir backend now auto-detects the replica source for its writes when pimdir.source is unset: it probes PimdirStore::distinct_sources() and, for a store synced as a single source (the local-sync case), writes as that one source — no configuration, no wrong-by-default local guard failure. It falls back to local only when the store has no or several sources.

Verified: with no pimdir.source configured, a \Seen edited in Himalaya against a one-side neverest store propagates to the remote on the next sync (the neverest report shows the push). Build/test/fmt clean.

Spec updated: backends (ADDED: pimdir writes auto-source).