feat: share env files as encrypted links

S → f on a repo, an env file or a key in one copies a
lazysecrets://share link holding the env files, encrypted (age) with
a secret's value: whoever can read that secret can open it. The key is
share.key in the repo's .lazysecrets.yaml, else in the config, else
picked when sharing; on Azure the link pins its version. Opening one
says what it holds, decrypts when asked, and saves the files in the
clone of the repo (or a folder chosen), replacing existing ones only
when asked. lazysecrets share prints a link from the command line.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

go.mod

@@ -29,6 +29,8 @@ require (
 	cloud.google.com/go/auth v0.18.2 // indirect
 	cloud.google.com/go/auth/oauth2adapt v0.2.8 // indirect
 	cloud.google.com/go/compute/metadata v0.9.0 // indirect
+	filippo.io/age v1.3.2 // indirect
+	filippo.io/hpke v0.4.0 // indirect
 	github.com/Azure/azure-sdk-for-go/sdk/internal v1.13.0 // indirect
 	github.com/Azure/azure-sdk-for-go/sdk/security/keyvault/internal v1.2.0 // indirect
 	github.com/AzureAD/microsoft-authentication-library-for-go v1.8.0 // indirect