Adds Posts::TagIndex (counts, per-tag posts, per-tag/year breakdown)
and Posts::TagReport, wired into a new `bake tags` task that prints a
frequency table plus a tag-by-year sparkline straight to the terminal.
Also generates /tags/ and /tags/<tag>/ HTML pages via the existing
Posts plugin, listing tags with post counts and per-tag post listings.
Not linked from nav yet -- reachable by URL only until tags are
curated enough to surface properly.
Extracted PostListItemView out of YearPostsView so the post-link
rendering (incl. link posts) is shared with the new tag pages instead
of duplicated.
Replace the Swift site generator with a Ruby and Phlex implementation.
Loads site and projects from TOML, derive site metadata from posts.
Migrate from make to bake and add standardrb and code coverage tasks.
Update CI and docs to match the new workflow, and remove unused
assets/dependencies plus obsolete tooling.
