Rename the Go module to sneak.berlin/go/secret (closes #43)

REPO_POLICIES.md requires the module root sneak.berlin/go/<name>.
go.mod, every import (rewritten with gofmt -r), the -X flags in
script/build and the examples in the pkg READMEs now use the new path.
go mod tidy lists go-humanize and fatih/color as direct requirements,
since internal/cli imports them.

This breaks anyone who fetched or imported git.eeqj.de/sneak/secret:
they must switch to sneak.berlin/go/secret, which resolves to this
repository.

Model: opus-5-5

internal/cli/usage_test.go

@@ -4,10 +4,10 @@ import (
 	"strings"
 	"testing"
 
-	"git.eeqj.de/sneak/secret/internal/cli"
-	"git.eeqj.de/sneak/secret/internal/secret"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/secret/internal/cli"
+	"sneak.berlin/go/secret/internal/secret"
 )
 
 // usageHeading starts the usage text cobra prints after an error.