Finish the move off weblorg

Remove what the migration left behind, then cut build.py down to the work
orgo does not do: 443 lines to 168.

- Delete publish.el and theme/templates/, superseded by content/orgo.toml
  and content/templates/. theme/static/ stays; orgo publishes it as assets.
- Drop the tag builder, the org-source copy, and the helpers that only fed
  them. orgo generates /tags/ and a page per tag from a collection.
- Rewrite the README and the migration-era comments that explained the
  config by pointing at what weblorg used to do.
- Serve styles.css as authored. Minifying 1KB of CSS saved 52 bytes over
  the wire once compressed, which did not pay for a build step and a
  toolchain dependency.
- Stop wiping the output directory. The .orgo-cache.json inside it is what
  makes a build incremental, and orgo already prunes outputs whose source
  is gone and re-emits any that are missing. Production keeps .build/ and
  development gets .build-dev/, because production rewrites image URLs in
  the output and a shared directory would let one environment reuse pages
  rendered for the other.

An incremental build is byte-identical to a clean one, and the site output
is unchanged apart from the /uses/ stack row.

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