PINW-40: safe Obsidian vault compatibility

[simonislee]

Summary

• adopt existing Obsidian vaults non-destructively while preserving .obsidian/ and unknown files
• add revision-checked atomic writes, checked rename/delete, bounded file history, and recovery
• cover Markdown, frontmatter, wikilinks, attachments, external conflicts, rename/delete, and Windows path forms with smoke tests
• document the filesystem compatibility contract and the features that remain Obsidian-owned

Verification

• npm run typecheck
• npm run test:mocks (132 files, 1,879 tests)
• npm run build
• MCP tests (22 tests; explicit file pattern under local Node 26 because the repo pins Node 20/npm 10)
• cargo test --locked --manifest-path src-tauri/Cargo.toml (406 passed, 2 ignored)

PINW-40