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>

schema/config-schema.json

@@ -70,6 +70,12 @@
             "items": { "type": "string" },
             "default": [],
             "description": "Branches that only sync mono → subrepo (never reverse)"
+          },
+          "exclude": {
+            "type": "array",
+            "items": { "type": "string" },
+            "default": [],
+            "description": "File/directory patterns to exclude from sync via josh :exclude filter. Josh pattern syntax: 'dir/' for directories, '*.ext' for globs, '**/dir/' for nested matches. Generates a .josh-filters/<target>.josh file that must be committed."
           }
         }
       }