feat(cli): agent-readable account list (reduced JSON view)

account list now routes to the agent role; an agent (EMCLI_KEY only) gets a
JSON envelope of name/from/can_send, while the admin keeps the full text
table. account add/edit/remove stay admin-only.

Also emit the agent path's missing-key/open failure as a JSON Failure
envelope (per spec), and update the stale run_test case that asserted the
old admin-only behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/cli/security_invariant_test.go

@@ -49,7 +49,7 @@ func TestAgentKeyCannotRunAdminCommands(t *testing.T) {
 
 	before := dbBytes(t, db)
 	adminAttempts := [][]string{
-		{"account", "list"},
+		{"account", "add", "--name", "x", "--imap-host", "h", "--username", "u@x.com"},
 		{"config", "set", "audit_retention_days", "30"},
 		{"audit"},
 	}