feat: parse and render Obsidian cross-references (task 0005)

Model `[[wikilinks]]` and `![[transclusions]]` as distinct inline node
types sharing a `{ target, anchor?, display? }` shape, and render them:
wikilinks underlined (display text or target), transclusions as their
raw source text.

remark-wiki-link is added to the pipeline for `[[…]]`. It does not
recognise `![[…]]` embeds or split the `#anchor` from the target, so
transclusions are recovered by scanning text runs and anchors are split
in the translation layer. Its alias divider is set to `|` so Step
Reference anchors like `#rolling:2` survive.

packages/core/package.json

@@ -13,6 +13,7 @@
     "remark-frontmatter": "^5.0.0",
     "remark-gfm": "^4.0.1",
     "remark-parse": "^11.0.0",
+    "remark-wiki-link": "^2.0.1",
     "unified": "^11.0.5",
     "yaml": "^2.9.0"
   }