Add site search (Pagefind)

Client-side index generated after the Hugo build; search box in the
header, results page with a section filter.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDRjtmAiNpo5mK5N3d7QwZ

README.md

@@ -29,10 +29,19 @@ banner, date, author and summary (`summary:` in the front matter, or the text be
 `<!--more-->`, or the first lines); all posts are listed under News. Posts are never touched by
 `make content`.
 
+## Search
+
+Search runs in the visitor's browser on an index generated by [Pagefind](https://pagefind.app/)
+at the end of `make build` (fetched by `npx`, so Node.js is needed to build). The header box
+sends to `/search/`, where results can be filtered by section. Content pages are indexed
+(`data-pagefind-body` on `<main>` in `layouts/baseof.html`); lists, taxonomy pages and the
+visitor-comments tabs are not. `hugo server` has no index: use `make build` and serve `build/`
+to try the search locally.
+
 ## Usage
 
     make content   # regenerate from ../oc (OC=path to use another location)
-    make build     # build into build/
+    make build     # build into build/, with the search index
     make check     # build + verify every internal link and image
     make serve     # preview on http://localhost:1313
     make deploy    # build and copy to the test server (share must be mounted; see DEPLOY_DIR/DEPLOY_URL)