Add Obsidian-friendly markdown vault export

[WalksWithASwagger]

Context

Legacy audio-transcription-studio documented an Obsidian-style knowledge vault with date folders, indexes, tags, wiki links, and deduped transcript outputs. Rebuild only the useful export shape in canonical WhisperForge.

Audit: docs/AUDIO-REPO-CONSOLIDATION-AUDIT-2026-05-18.md

Acceptance Criteria

• Define a markdown vault export format for captures, transcripts, recaps, and generated outputs.
• Preserve source receipts, run IDs, recipe metadata, tags, and links back to related captures/runs.
• Avoid duplicating existing markdown export behavior unless a vault mode is explicitly selected.
• Document the directory/index/tag conventions.
• Add tests for serialization and path generation.

Verification

• make test
• git diff --check

[WalksWithASwagger]

Shipped in 2a7e658. Added Obsidian-friendly vault export support on top of the existing markdown export, with date-organized notes, metadata preservation, source receipts, and a lightweight vault index. Verification passed: venv/bin/python -m pytest tests/test_captures.py tests/test_resurfacing.py tests/test_export.py -q; make test; make eval-fixture; venv/bin/python tests/ui_smoke.py; make smoke; python3 -m json.tool ops/roadmap/features.json; git diff --check.