gitignore: cover debhelper-generated dirs and stray .deb artifacts

The original .gitignore caught *.deb in the source tree but not:
- debian/.debhelper/  (cache populated by every dh_* helper)
- debian/debhelper-build-stamp, debian/postit.debhelper.log,
  debian/postit.substvars (regenerated per build)
- debian/tmp/         (staging tree for dh_install)
- postit_*.{deb,changes,buildinfo} in the parent workspace
  (where dpkg-buildpackage drops artifacts)

.gitignore

@@ -14,3 +14,17 @@ build/
 .idea/
 .vscode/
 *.swp
+# Generated by debhelper during build
+debian/.debhelper/
+debian/debhelper-build-stamp
+debian/postit.debhelper.log
+debian/postit.substvars
+
+# Staging tree for dh_install
+debian/tmp/
+
+# Built .deb artifacts that fall outside the source tree (dpkg-buildpackage
+# drops them next to the parent dir)
+postit_*.deb
+postit_*.changes
+postit_*.buildinfo