chore: gitignore _build/; correct CITATION author email

Add _build/ to .gitignore (Agda build artifacts were untracked but
not ignored). Change CITATION.cff author email from
jonathan.jewell@gmail.com to j.d.a.jewell@open.ac.uk to match the
rest of the repo (CHANGELOG, SPDX headers, etc.).

https://claude.ai/code/session_01BwV2DWsjkBiNP3oscimMLV

Author: claude

.gitignore

@@ -1,4 +1,5 @@
 *.agdai
+_build/
 .claude/
 .agda/
 

CITATION.cff

@@ -7,7 +7,7 @@ type: software
 authors:
   - given-names: Jonathan
     family-names: Jewell
-    email: jonathan.jewell@gmail.com
+    email: j.d.a.jewell@open.ac.uk
     # PLACEHOLDER: insert ORCID once registered, e.g.
     # orcid: "https://orcid.org/0000-0000-0000-0000"
 repository-code: "https://github.com/hyperpolymath/echo-types"