Add export/install to source-manage.js: pack a source (config entry +
recipe + fixtures) into a single self-contained .nhsource.json, install
on another machine in one command. Bundle excludes machine-specific
fields (vaultPath/enabled/articleUrlPattern); --force overwrites on id
conflict (matches the helper command convention). Path-traversal guarded.

Includes bundles/nikkei-tech.nhsource.json as a ready-to-install example
(the bundle itself carries the recipe + fixtures; the unpacked
helpers/nikkei-tech.* files stay out of this commit).

Documents the flow in SKILL.md (Quick Reference + Sharing sources) and
references/source-management.md.