test: add E2E tests for vault/sync endpoints (#12)

[VECTORG99]

Obsidian Sync E2E Tests (#12)

Changes

• api/tests/test_e2e_vault.py (NEW) — Vault/sync endpoints and Obsidian import flow:
  • test_vault_write_daily_returns_ok — POST /vault/write-daily
  • test_vault_write_objectives_returns_ok — POST /vault/write-objectives
  • test_vault_write_skills_returns_ok — POST /vault/write-skills
  • test_create_note_with_obsidian_source — Note creation with source: obsidian
  • test_imported_note_found_by_source_path — GET /notes/ filtered by source_path
  • test_update_imported_note_content — PUT note mutated from vault
  • test_rebuild_derived_data — POST /notes/rebuild-derived
  • test_tag_graph_after_import — GET /tags/graph post-import

Stacking

Branch base: feat/11-e2e-tests (needs pytest infrastructure + middleware)

Closes #12