feat: pick the share key in Go to, or by reference

Sharing without share.key opens Go to as a picker of secrets instead
of asking for a link: it lists only secrets, enter picks one, and a
pasted reference picks it too. The key, there and in share.key or
--key, can be a secret's lazysecrets:// link, Key Vault URL or
@Microsoft.KeyVault(...) reference.

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

docs/repos.md

@@ -60,7 +60,7 @@ A repo's `.lazysecrets.yaml` can also name the secret its env files are
 
 ```yaml
 share:
-  key: lazysecrets://azure/kv-team.vault.azure.net/secrets/env-share
+  key: https://kv-team.vault.azure.net/secrets/env-share   # or its lazysecrets:// link
 ```
 
 ## Decrypting