migrate-posts-to-v2-frontmatterlisted

# Migrate Posts to v2 Frontmatter ## Coverage - The four-phase pattern for adding a required field to a populated content tree — *add as nullable → backfill from existing data → verify → flip the validator to require it* — and why collapsing any two phases into one is unsafe - The backfill query — generating a `summary` from each post's first paragraph, with a per-post manual-override fallback for cases where the auto-summary is wrong - The verification gate between backfill and validator-flip — running `validate-posts.ts` against the entire `content/posts/**/*.md` tree must return zero errors before the schema is updated - The tag-normalization step — mapping every tag to its canonical form in `tag-vocabulary.ts`, with a deny-list for tags that should be removed entirely (e.g., legacy synonyms now folded into a canonical tag) - The dry-run gate — the migration script always runs in dry-run by default, printing the diff per post; the `--apply` flag is opt-in and never the CI default - The rollback path — what `ROLLBACK.md` for this migration looks like and why "regenerate every summary" is wrong (overwrites authored summaries); the correct rollback restores the per-post `.bak` file the migration writes alongside each edit ## Philosophy A content-schema migration is a rare migration where being careful is cheaper than being clever. The temptation to combine the four phases into one "atomic" pass fails because the backfill produces some surprising auto-summaries, the human