iter_skills_files() walked the skills tree with a bare rglob("*"), so the
.hub download cache, .archive, curator backups, and any node_modules/.git
under a skill package were uploaded to the sandbox on every sync. The
sandbox never reads them: skill content is resolved host-side.
EXCLUDED_SKILL_DIRS is already the canonical exclusion set, honoured by
discovery and backup. Apply it to the sync path too, across all three
roots iter_skills_files() walks (local, external, project-local), and add
.curator_backups to the set.
Measured on a local install: 900 files / 67.3 MB -> 771 files / 8.4 MB.
This is not just wasted bandwidth on the SSH backend, where the oversized
payload can exceed the 120s _ssh_bulk_upload deadline and surface as the
agent hanging on every tool call.
The filter intentionally does not reuse is_excluded_skill_path(), which
also prunes references/, templates/, assets/ and scripts/ -- those hold
support files and bundled scripts the sandbox does read and execute.
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