fix: resolve markdown-style relative .md links to permalinks

[oleeskild]

Problem

Gardens whose notes use plain markdown links ([X](../concepts/x.md)) instead of wikilinks end up with broken links and an empty graph:

• The link pipeline (plugin convertLinksToFullPath, the link filter, link_open rule) only handles [[wikilinks]], so relative .md hrefs reach the published HTML untouched.
• Pages are served at trailing-slash URLs, so the browser resolves ../concepts/x.md one directory too deep: /wiki/concepts/concepts/x.md → 404 (reported by a user with exactly this duplicated segment).
• extractLinks only matches wikilinks and root-absolute hrefs, so the graph shows isolated dots with no edges and backlinks are empty.

Fix

• src/helpers/linkUtils.js
  • resolveVaultPath(target, sourceDir): resolves relative/root-anchored link targets against the note's vault directory; rejects external schemes and paths escaping the vault.
  • convertMdHrefs(html, sourceDir, resolveAnchor): rewrites .md hrefs in rendered HTML, preserving fragments.
  • extractLinks(content, sourcePath): now also extracts markdown-style .md links (image embeds excluded) resolved to vault stems, so the graph and backlinks pick them up.
• .eleventy.js — new resolveMdLinks filter that derives the note's vault directory from its inputPath and reuses getAnchorAttributes, so unresolved targets get the same /404 handling as dead wikilinks. Accepts an explicit input path for looped contexts.
• note.njk / index.njk / feed.njk: apply the filter; the feed passes note.inputPath explicitly since this.page there is the feed template, not the note.