feat: dependency graph improvements

[d12frosted]

Summary

More sophisticated dependency tracking for smarter incremental builds.

Vulpea Integration

• Use vulpea link graph to understand note relationships
• "Note A embeds content from B" → A depends on B (hard dependency)
• "Note A links to B" → soft dependency (rebuild A if B's title/slug changes)

Output-Level Tracking

• Currently: note-level cache
• Target: output-level cache (a note may produce multiple outputs)

Parallel Builds

• Build independent outputs concurrently
• Respect dependency ordering
• Configurable parallelism level

Open Questions

• How to express "soft" vs "hard" dependencies in cache?
• Cycle detection in dependency graph?

Dependencies

• feat: server mode with incremental builds #7 (server mode — incremental builds need dependency tracking)