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

script/build

@@ -17,7 +17,7 @@ main() {
             echo dev)"
     fi
     commit="$(git rev-parse HEAD 2>/dev/null || echo unknown)"
-    pkg=git.eeqj.de/sneak/secret/internal/cli
+    pkg=sneak.berlin/go/secret/internal/cli
     # Build the file, not the package `./cmd/secret`: a package build
     # also stamps git status into the binary and fails where git cannot
     # read the checkout, instead of falling back to `dev`/`unknown`.