* Content: import prosemirror-markdown directly (drop @tiptap/pm/markdown re-export)

@tiptap/pm/markdown is a thin re-export of prosemirror-markdown but is
gated by tiptap's peer-dep version resolution, which made the
defaultMarkdownSerializer reference flaky across pnpm's hoisted
graph (bundlers ended up with two prosemirror-markdown copies and the
override on .nodes.image only patched one of them).

Add prosemirror-markdown as a direct dep on the content template (plus
yjs for the same hoisting reason) and switch the imports in
VisualEditor.tsx and ImageNode.ts. Lockfile regenerated.

* chore: bump @agent-native/core to 0.7.17