chore(gitattributes): union-merge for append-only board files

AGENT_LOG / EPIPHANIES / INTEGRATION_PLANS are PREPEND-ONLY per the
CCA2A pattern (CLAUDE.md mandatory board-hygiene rule). When two
branches concurrently prepend entries, the right semantic resolution
is to keep both — there is no overwrite by construction. Declaring
`merge=union` avoids manual conflict resolution on every rebase.

https://claude.ai/code/session_017gZ6sPRXYPj5n7uJ7NBtRv

Author: claude

.gitattributes

@@ -0,0 +1,6 @@
+
+# Append/prepend-only governance files use union merge so concurrent
+# prepends from different branches don't conflict (per CCA2A pattern).
+.claude/board/AGENT_LOG.md         merge=union
+.claude/board/EPIPHANIES.md        merge=union
+.claude/board/INTEGRATION_PLANS.md merge=union