Add file exclusion via josh stored filters (v1.2.0)

New `exclude` config field per target generates .josh-filters/<name>.josh
files with josh :exclude clauses. Josh-proxy applies exclusions at the
transport layer — excluded files never appear in the subrepo.

Preflight checks that generated filter files are committed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

VERSION

@@ -1 +1 @@
-1.1.0
+1.2.0