debian/rules: skip dh_strip on *.so (cross-compiled native libs)

When cross-compiling PostIt.Desktop for linux-arm64 from an amd64
runner container, the .deb ends up shipping HarfBuzzSharp.so and
SkiaSharp.so in ARM64 format. dh_strip invokes objcopy on every
file under debian/postit/, and objcopy on the amd64 host cannot
parse ARM64 .so files:

  objcopy: Unable to recognise the format of the input file
  `debian/postit/usr/lib/postit/libHarfBuzzSharp.so'
  dh_strip: error: ... returned exit code 1
  dh_strip: error: Aborting due to earlier error

Add --exclude=*.so to override_dh_strip. The upstream .so ships
with its own debug symbols; we don't lose anything by skipping
the strip on them.

Also drop the diag.yml workflow that diagnosed the initial
tag-not-found failure — no longer needed.

Discovered via the Forgejo Actions release.yml run on tag 1.0.6.

debian/rules

@@ -129,7 +129,20 @@ override_dh_shlibdeps:
 	dh_shlibdeps
 
 override_dh_strip:
-	dh_strip --exclude=.pdb
+	# --exclude=.pdb : .NET génère des .pdb même en Release, ils
+	# ne sont pas strippables proprement.
+	# --exclude=*.so  : Avalonia dépend de HarfBuzzSharp et SkiaSharp
+	# qui livrent leurs .so pré-compilés via NuGet (formats natifs
+	# multi-arch : linux-x64, linux-arm64, etc.). Quand on cross-
+	# compile depuis un hôte amd64 (build arm64), dh_strip appelle
+	# objcopy sur ces .so qui sont en ARM64 et objcopy natif amd64
+	# ne sait pas les lire :
+	#   objcopy: Unable to recognise the format of the input file
+	#   `debian/postit/usr/lib/postit/libHarfBuzzSharp.so'
+	# On les exclut donc du strip. Les symboles debug seront
+	# présents dans le .deb mais ce sont des libs upstream, pas
+	# notre code — pas critique.
+	dh_strip --exclude=.pdb --exclude=*.so
 
 
 define stage_postit