infra: cross-platform support and hub-and-spoke mirroring

.gitattributes normalises line endings to LF for cross-machine
Linux/Windows work. .agdai marked binary.

Mirror workflow if present handles GitLab and Codeberg via
GIT_ASKPASS auth; missing-token cases silently skip rather than
fail the build.

Author: hyperpolymath

.gitattributes

@@ -0,0 +1,9 @@
+* text=auto eol=lf
+*.agda text eol=lf
+*.adoc text eol=lf
+*.md text eol=lf
+*.yml text eol=lf
+*.yaml text eol=lf
+*.scm text eol=lf
+*.a2ml text eol=lf
+*.agdai binary