#724 extension source: install skills from source-path extensions

When an extension is pulled from the registry and its manifest.yaml declares a skills field, swamp installs the skill files into .claude/skills/<name>/ in the consuming repo. Confirmed working with @magistr/good-planning.

Source-path extensions (loaded via .swamp-sources.yaml) don't get the same treatment. The extension source add --only enum is models,vaults,drivers,datastores,reports,workflows — skills are absent. swamp repo upgrade also doesn't sync skills from source paths.

Desired behavior: Skills declared in a source-path extension's manifest.yaml should be installed into the repo's configured skills directory (.claude/skills/ for standard Claude repos, or the custom skillsDir from .swamp-custom-tools.yaml) on swamp repo upgrade or a new extension source sync command — matching what extension pull already does.

Context: @example/my-extension is a private extension loaded from source path in multiple repos. Adding a skill (my-workflow) to its manifest and expecting it to propagate to consuming repos surfaces this gap.