feat: share.keyRules, how long and varied a share key must be

Sharing refuses a key shorter than minLength characters (default 32,
never under 16) or of fewer than minClasses kinds of lowercase,
uppercase, digits and symbols (default 2, so random hex passes),
saying so without showing it. Opening doesn't check.

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

internal/config/config_test.go

@@ -72,6 +72,9 @@ func TestLoadRejectsMistakes(t *testing.T) {
 		"view":       "sourcesView: tree\n",
 		"releases":   "releases: [git.example.com/team/lazysecrets]\n",
 		"http":       "releases: [http://git.example.com/team/lazysecrets]\n",
+		"share key":  "share:\n  key: https://kv.vault.azure.net/\n",
+		"short keys": "share:\n  keyRules:\n    minLength: 8\n",
+		"classes":    "share:\n  keyRules:\n    minClasses: 5\n",
 	} {
 		if _, err := Load(write(t, content)); err == nil {
 			t.Errorf("%s: no error", name)