0.2.0

Release v0.2.0: plugin hosting, graph view, unlinked mentions

Bundles the four merged PRs into a release:
- #5 Plugin API layer (geode module, PluginManager, Component/Plugin)
- #8 Host real Obsidian plugins — Claude Threads runs in Geode
- #6 Unlinked mentions + backlink context snippets
- #7 Global graph view

Also raises GEODE_API_VERSION to 1.8.0 so it represents the Obsidian-API
compatibility level plugins gate against (Claude Threads needs 1.0.0),
distinct from Geode's own 0.2.0 app version.