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

pkg/bip85/README.md

@@ -22,8 +22,8 @@ BIP85 enables a variety of use cases:
 ```go
 import (
     "fmt"
-    "git.eeqj.de/sneak/secret/pkg/bip85"
     "github.com/btcsuite/btcd/btcutil/hdkeychain"
+    "sneak.berlin/go/secret/pkg/bip85"
 )
 
 // Parse an existing master key
@@ -147,7 +147,7 @@ test vectors for the DRNG functionality.
 Run the tests with verbose output to see the test vectors and results:
 
 ```
-go test -v git.eeqj.de/sneak/secret/pkg/bip85
+go test -v sneak.berlin/go/secret/pkg/bip85
 ```
 
 ## References