feat: add Obsidian vault ingestion

[raoabinav]

Evaluator Summary

• Abi added an Obsidian vault ingestion example that indexes Markdown notes while preserving vault-aware metadata such as relative paths, titles, aliases, tags, wikilinks, embeds, and frontmatter.
• The design treats Obsidian as a graph-shaped local knowledge base, so retrieval results carry enough metadata for backlink-aware and note-aware follow-ups.
• The implementation is scoped to Markdown vault ingestion rather than an Obsidian plugin or live sync, keeping the first PR reviewable and testable.
• Quality bar: focused tests cover vault parsing and metadata extraction; lint/format checks were run for the app and tests.

[raoabinav]

Parking this PR for now to keep the review queue focused on #347. I’ll revisit/reopen this work later as a smaller clean PR once the content-hash PR lands.