feat(newsletter): port the engine to JavaScript

Translates the seven-subcommand engine to Node ESM, one module per former Go
file, invoked as `node scripts/newsletter <command>` from the repo root. The Go
implementation stays in place for now so parity can be measured against it.

Hand-rolled HTML and XML regexes give way to cheerio, which removes the manual
string surgery in caption extraction and covers RSS and sitemap XML through
xmlMode without a second parser. fetch-via-defuddle gains a local extraction
stage ahead of the defuddle.md proxy; the proxy stays, because fetching from a
third IP is the whole point when this machine's IP is the blocked one. Both
stages now emit the same YAML frontmatter plus body, and an extraction that
looks like a bot challenge counts as a local failure so the proxy still runs.

Behaviour is preserved where it is load-bearing rather than where it is merely
idiomatic: query strings are rebuilt by string surgery so surviving parameters
keep their original order and encoding, duplicate detection stays an index loop
with byte-offset boundary checks so a prefix of a stored URL is not a false
match, empty optional fields are omitted rather than emitted as "", a deep-crawl
miss reports cutoff as null rather than dropping the key, bullet filtering counts
code points, tag counts keep their six-column alignment, and a malformed percent
sequence falls back to the raw substring.

The printer lives in its own module so a command module never imports the
dispatcher: index.js runs main() at module scope, so that cycle would execute
the CLI as a side effect of any import. Missing dependencies report one
actionable line instead of a module-resolution stack trace, and a reader that
closes early exits quietly instead of raising EPIPE.

package.json

@@ -9,9 +9,13 @@
   },
   "scripts": {
     "lint": "eslint .",
-    "projects:refresh": "node .github/scripts/update-projects-list.js"
+    "projects:refresh": "node .github/scripts/update-projects-list.js",
+    "test": "node --test scripts/newsletter/*.test.js"
   },
   "dependencies": {
+    "cheerio": "^1.2.0",
+    "defuddle": "^0.19.4",
+    "linkedom": "^0.18.13",
     "octokit": "^5.0.5"
   },
   "devDependencies": {