chore: update maintainers and add CODEOWNERS (#1950)

## Summary

- Move Tom Wilkie to Emeritus section in MAINTAINERS.md
- Add Jay DeLuca (@jaydeluca) as a maintainer
- Add CODEOWNERS file listing active maintainers (@fstab, @dhoard,
@zeitlinger, @jaydeluca) for automated review assignment

## Test plan

- [ ] Verify MAINTAINERS.md content is correct
- [ ] Verify CODEOWNERS file is recognized by GitHub after merge

Author: zeitlinger

CODEOWNERS

@@ -0,0 +1,4 @@
+# https://help.github.com/articles/about-codeowners/
+# https://git-scm.com/docs/gitignore#_pattern_format
+
+* @fstab @dhoard @zeitlinger @jaydeluca

MAINTAINERS.md

@@ -2,5 +2,9 @@
 
 - Fabian Stäber <fabian@fstab.de> @fstab
 - Doug Hoard <doug.hoard@gmail.com> @dhoard
-- Tom Wilkie <tom@grafana.com> @tomwilkie
 - Gregor Zeitlinger <gregor.zeitlinger@grafana.com> @zeitlinger
+- Jay DeLuca <jaydeluca4@gmail.com> @jaydeluca
+
+## Emeritus
+
+- Tom Wilkie <tom@grafana.com> @tomwilkie