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/agehd/README.md

@@ -43,7 +43,7 @@ import (
     "fmt"
     "log"
 
-    "git.eeqj.de/sneak/secret/pkg/agehd"
+    "sneak.berlin/go/secret/pkg/agehd"
 )
 
 func main() {
@@ -69,7 +69,7 @@ import (
     "fmt"
     "log"
 
-    "git.eeqj.de/sneak/secret/pkg/agehd"
+    "sneak.berlin/go/secret/pkg/agehd"
 )
 
 func main() {
@@ -95,7 +95,7 @@ import (
     "fmt"
     "log"
 
-    "git.eeqj.de/sneak/secret/pkg/agehd"
+    "sneak.berlin/go/secret/pkg/agehd"
 )
 
 func main() {
@@ -122,7 +122,7 @@ import (
     "fmt"
     "log"
 
-    "git.eeqj.de/sneak/secret/pkg/agehd"
+    "sneak.berlin/go/secret/pkg/agehd"
 )
 
 func main() {