ANG-011:Incremental updates via workspace events

[yugalkaushik]

1. Live updates without reopening the panel

• Listens to Joplin workspace events: note changed, note selected, sync completed
• When you edit a note, the graph updates automatically after a short delay instead of needing a manual refresh

2. Two ways of detecting what changed

• Uses Joplin's /events API with a saved cursor to catch new, updated, and deleted notes
• When AI is on, also sweeps the getEmbeddings API with its own saved cursor to catch notes that got re-embedded
• If AI is off, it just relies on /events for everything

3. Graph is now cached on disk

• The built graph itself (not just embeddings) is saved to a local SQLite database
• Reopening the panel loads instantly from this cache instead of recomputing the whole graph from scratch

4. Partial graph updates instead of full rebuilds

• Added a diffing step that compares the old graph to the new one and figures out exactly which nodes and edges were added, changed, or removed
• The webview applies just that small patch instead of tearing down and rebuilding the whole graph
• Nodes that weren't touched keep their position on screen instead of jumping around on every edit

5. Small logic changes to support all this

• Tag edges now get a stable, order independent identity so they don't flicker during partial updates
• The graph builder now assigns explicit IDs to edges so the diffing step can match them up between rebuilds