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/config.md

@@ -36,7 +36,7 @@ providers:
     include: ["secrets/**"]
     exclude: ["**/testdata/**"]
 share:                 # sharing env files as encrypted links
-  key: lazysecrets://azure/kv-team.vault.azure.net/secrets/env-share  # for repos naming none
+  key: https://kv-team.vault.azure.net/secrets/env-share  # or its lazysecrets:// link; for repos naming none
 ```
 
 More, each on its own page: [repos](repos.md) (`providers.repos`),