[pull] main from tobi:main - #39

Allow QMD to be used as a library (`import { createStore } from '@tobilu/qmd'`)
in addition to CLI and MCP modes. The constructor requires explicit dbPath and
either a configPath (YAML file) or inline config object — no defaults assumed,
making it safe to embed in any application.

- Add src/index.ts entry point with QMDStore interface exposing search,
  retrieval, collection/context management, and index health
- Add setConfigSource() to collections.ts for inline config support
  (in-memory config with no file I/O)
- Add main/types/exports fields to package.json
- Add SDK documentation section to README
- Add 56 unit tests covering constructor, collections, contexts, search,
  document retrieval, config isolation, YAML persistence, and lifecycle