Drivers: harden provider drivers and third-party conformance

[FerdinandSu]

Scope

Track provider driver quality, compatibility, caching, and third-party driver readiness.

Checklist

• Harden WebDAV support against common server variations.
• Improve Git provider support with branches, tags, and commit metadata.
• Expand Zotero attachment handling and indexing support.
• Add cache retention and encrypted cache options.
• Add conformance tests for third-party drivers.

Acceptance Criteria

• WebDAV behavior is tested against representative server variations.
• Git-backed drivers expose branch/tag/commit metadata consistently.
• Zotero can expose useful metadata and attachment-backed content when configured.
• Cache policy behavior is observable and testable.
• External driver authors have a conformance test path.

Source

Roadmap section: docs/roadmap.md / Drivers.