Expose unresolvedLinks in document metadata (#305)

Adds an unresolvedLinks field to NoteJson (surfaced via GET with
Accept: application/vnd.olrapi.note+json, the vault_read MCP tool,
and search_query's JsonLogic evaluation context), sourced from
Obsidian's metadataCache.unresolvedLinks. Lets clients find links
that point at notes which don't exist yet.

Fixes #302

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>