pimdir backend auto-sources its writes

Why

The pimdir backend attributes staged writes to a replica source (pimdir.source), which for propagation must match the source the sync drives. Requiring the user to know and set that name was a wart — and wrong-by-default (local), which failed loudly on a store synced as left.

What

When pimdir.source is unset, the backend auto-detects it: it reads the store's distinct_sources() and, for a store synced as a single source (the local-sync case), writes as that one source — no configuration. It falls back to local only when the store has no or several sources.

Verified: with no pimdir.source, a flag edited in Himalaya against a one-side neverest store propagates to the remote on the next sync.

Scope / non-goals

• Multi-source stores still need an explicit pimdir.source (or fall back to local); the clean case is the single-source local cache.