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

internal/config/config.go

@@ -56,7 +56,7 @@ var (
 	errRecipientNotX25519 = errors.New(
 		"not a valid recipient; only X25519 age1... public keys are supported")
 	errNoSnapshots = errors.New(
-		"at least one snapshot must be configured (see config.example.yml)")
+		"at least one snapshot must be configured (see configs/config.example.yml)")
 	errSnapshotNoPaths   = errors.New("snapshot must have at least one path")
 	errChunkSizeTooSmall = errors.New("chunk_size must be at least 1MB")
 	errBlobSizeTooSmall  = errors.New(