Unify lifecycle flow across setup/adopt/follow/skim/vet
- add config.requireDid and vit-dir readFollowing/writeFollowing helpers\n- remove FOLLOW_COLLECTION from constants\n- setup: add git/bun prerequisite checks and login guidance\n- adopt: stop initializing .vit and guide user to run vit init via agent\n- follow/unfollow/following: switch to project-local .vit/following.json\n- skim: read follows from following.json and print ref/title/description with vet hint\n- vet: accept 3-word ref input and resolve newest matching cap via following + beacon\n- standardize human gate messages on setup/adopt/vet\n- update related tests and skills/vit/SKILL.md for new lifecycle