Markdownify MCP

Path-resolution overhaul to make Docker (and any non-default install layout) frictionless. Closes #97 via #98.

What's new

• MARKITDOWN_PATH env var. Defaults to <project>/.venv/bin/markitdown, falls back to a bare-name PATH lookup. Lets pipx install "markitdown[pdf]" or any system install work without a project venv.
• REPOMIX_PATH env var. Same shape, for git-repo-to-markdown.
• MD_ALLOWED_PATHS env var (path-delimiter-separated). Enforced on every filepath-accepting tool, not just get-markdown-file. MD_SHARE_DIR kept as a single-dir backward-compat alias.
• Docker volume-mount example in the README, aligned with the markdownify.paths field on the Docker MCP catalog config page.
• New scripts/docker-smoke-test.sh: builds the image, mounts a sample dir, calls pdf-to-markdown over the MCP stdio transport, asserts the response.

Fixes

• Stop treating non-empty stderr from markitdown as fatal — it emits non-fatal warnings on every successful run (onnxruntime cpuid, pydub ffmpeg-missing). Exit code is the real signal.
• Add .dockerignore so COPY . . no longer drags the host's .venv and node_modules into the build context.
• Replace stale UV_PATH docs (the code stopped using uv several versions ago).

Breaking changes

• projectRoot removed from the MCP tool input schema. It was client-controllable, so any MCP client could force the server to exec <arbitrary>/.venv/bin/markitdown. No legitimate client should have been using it. If you need to override the project root, set MARKITDOWN_PATH directly.