feat: reddit -> spotify/bandcamp artist linker MVP (extension + resolver service)

Firefox MV2 content script that detects artist/band names in reddit comment
threads and rewrites them inline into Spotify + Bandcamp links, backed by a tiny
zero-dependency Node resolver that confirms each candidate against the Spotify Web
API (direct links) or MusicBrainz (zero-setup, search links). Includes build +
install instructions in the README.

Source: project/reddit-spotify-linker (user 2026-06-12) + user message 2026-06-14
"Full vibe code, small service, minimal code."

Closes task/resolver-service, closes task/firefox-extension, closes task/docs-and-publish
(plan/mvp-build, objective/ship-mvp-linker).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extension/styles.css

@@ -0,0 +1,23 @@
+/* Injected links — kept subtle so reddit comments stay readable. */
+.rsl-link {
+  text-decoration: underline dotted #1db954 !important; /* spotify green */
+  text-underline-offset: 2px;
+  cursor: pointer;
+}
+.rsl-link:hover {
+  text-decoration: underline solid #1db954 !important;
+}
+.rsl-sup {
+  font-size: 0.7em;
+  margin-left: 1px;
+  vertical-align: super;
+  line-height: 0;
+}
+.rsl-bc {
+  color: #629aa9 !important; /* bandcamp teal */
+  text-decoration: none !important;
+  font-weight: 600;
+}
+.rsl-bc:hover {
+  text-decoration: underline !important;
+}