refactor(source-control): derive the floor's lane-plugin list and keep repo paths out of the plugin default #8279

From the round 62 assumption sweep (S28, S29).

Lane-plugin list is hand-maintained

docs/conventions/pr-pipeline.yaml floor-paths lists seven lane-loaded plugins by hand. The lanes load plugins derived from each activity's skill plus discovery and knowledge (.github/actions/collect-base-activity/action.yml:161,233). A new activity skill from another plugin leaves its scripts outside the floor.

Derive the list from the activity config instead.

Repo paths in a plugin default

babysit_risk.py's code floor hard-codes this repository's own paths (for example .work-item-tracker.json and the plugins/disk-hygiene guard scripts) into a plugin default. Consumer repositories also run that default and cannot remove those entries (C14: plugin defaults are fallbacks).

Move the repo-specific entries to this repo's config.

Done when

• The lane-plugin floor list comes from one source.
• The plugin's built-in floor holds only paths every consumer shares.