bibliography: citar + biblio + org-cite CSL setup

- init.el: biblio module with +citar flag
- packages.el: citeproc for org-cite CSL export
- config.el: citar (browse/insert/notes), biblio (online search),
  org-cite with CSL processor, bibtex editing defaults
- Keybindings under SPC b: browse, insert, notes, search online
- Auto-creates ~/org/references.bib if missing

packages.el

@@ -109,6 +109,9 @@
 (package! combobulate :recipe (:host github :repo "mickeynp/combobulate"))
 (package! breadcrumb :recipe (:host github :repo "joaotavora/breadcrumb"))
 
+;; Bibliography: CSL citation processor for org-cite export
+(package! citeproc)
+
 ;; Tier 3 — Supplementary
 (package! org-anki)
 (package! org-ql)