Refactor server config: single --config flag replaces --secrets/--mail-perms

- New server/config.mjs loads config.json, resolves secrets path relative
  to config dir, returns users/smtp/mail_perms_path/bind/port
- server/secrets.mjs removed (logic absorbed into config.mjs)
- smtp moves from secrets.json to config.json
- secrets.json now contains only users (pure credentials)
- config.example.json added as reference template
- .gitignore/.npmignore updated to cover config.json and mail-perms.json
- README updated with new setup and flags

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config.example.json

@@ -0,0 +1,13 @@
+{
+	"secrets": "../secrets.json",
+	"mail_perms": "mail-perms.json",
+	"smtp": {
+		"host": "smtp.example.com",
+		"port": 587,
+		"secure": false,
+		"auth": { "user": "relay@example.com", "pass": "<password>" },
+		"from": "agent@example.com"
+	},
+	"bind": "127.0.0.1",
+	"port": 3015
+}