Add #rss command family with LLM tags and summaries (v0.20.0)

#rss subcommands on the command channel: help / add <url> <name> /
remove <name> / tags / sources / get <name|#tag>. Sources are stored in
a new rss_sources table in the existing MESH_DB_PATH file (second
node:sqlite connection, case-insensitive names); feeds parsed with
fast-xml-parser (RSS 2.0 + Atom); AI tags and headline summaries via
injected LlmService.chat() with bounded maxTokens (50/200).

CommandService gains prefix registration so one handler owns the whole
#rss family while #ping stays an exact match; wired in CommandModule's
factory — still no constructor-level cycle, no forwardRef. One dead feed
does not sink a tag summary.

src/rss/rss.feed.ts

@@ -0,0 +1,60 @@
+import { XMLParser } from 'fast-xml-parser';
+
+/** One feed item: title always present (non-empty), link where the feed provides one. */
+export interface FeedItem {
+  title: string;
+  link?: string;
+}
+
+const parser = new XMLParser({ ignoreAttributes: false });
+
+function asText(value: unknown): string | undefined {
+  if (typeof value === 'string') return value.trim() || undefined;
+  // element with attributes/CDATA parses to an object holding its text under #text
+  const t = (value as Record<string, unknown>)?.['#text'];
+  if (typeof t === 'string' && t.trim()) return t.trim();
+  return undefined;
+}
+
+function asLink(value: unknown): string | undefined {
+  const fromText = asText(value);
+  if (fromText) return fromText; // RSS 2.0 <link>url</link>
+  // Atom <link rel="alternate" href="..."/> — the URL is an attribute
+  const href = (value as Record<string, unknown>)?.['@_href'];
+  if (typeof href === 'string' && href.trim()) return href.trim();
+  return undefined;
+}
+
+function collect(value: unknown): Record<string, unknown>[] {
+  // a single item/entry parses to an object, several to an array
+  if (!value) return [];
+  const arr = Array.isArray(value) ? value : [value];
+  return arr.filter(
+    (v): v is Record<string, unknown> => !!v && typeof v === 'object',
+  );
+}
+
+/**
+ * Parse RSS 2.0 (`<item><title>/<link>`) or Atom (`<entry><title>`) XML into items with
+ * non-empty titles, in document order (feeds are newest-first). Returns [] for malformed
+ * input or feeds without usable items — that empty result IS the "not a valid feed" signal.
+ */
+export function parseFeed(xml: string): FeedItem[] {
+  let doc: unknown;
+  try {
+    doc = parser.parse(xml);
+  } catch {
+    return []; // fast-xml-parser is lenient, but a hard failure means no items either
+  }
+  const root = doc as Record<string, any>;
+  // coalesce the RAW values first — collect() always returns an array, so `collect(a) ?? collect(b)` could never fall through to b
+  const rawItems = collect(root?.rss?.channel?.item ?? root?.feed?.entry);
+  const items: FeedItem[] = [];
+  for (const item of rawItems) {
+    const title = asText(item?.title);
+    if (!title) continue; // skip entries without a usable title
+    const link = asLink(item.link);
+    items.push(link ? { title, link } : { title });
+  }
+  return items;
+}