Move documents to docs/ and the example config to configs/ (closes #411)

ARCHITECTURE.md and TODO.md move to docs/, config.example.yml to
configs/, as REPO_POLICIES.md asks of the root. Links, the
no-snapshots error message and the example-config test follow them.

Model: opus-5-5

docs/REPOSTRUCTURE.md

@@ -304,7 +304,7 @@ so an operator can weigh them rather than discover them.
 4. **A snapshot is marked complete in the local DB only after its metadata is
    uploaded** - `finalizeSnapshotMetadata` runs `ExportSnapshotMetadata` first
    and records completion (`MarkSnapshotComplete`) only once the export succeeds
-   (see the backup data flow in [ARCHITECTURE.md](../ARCHITECTURE.md)). A crash
+   (see the backup data flow in [ARCHITECTURE.md](ARCHITECTURE.md)). A crash
    during the export therefore leaves the snapshot incomplete, so the next
    backup's `PruneDatabase` drops it and re-backs-up its data, rather than
    leaving a completed-looking row in the local index with no matching metadata