Add Nix flake packaging (repomap + repomap-mcp)

[kremovtort]

Summary
This PR adds first-class Nix flake support for RepoMapper, making the project easy to build/run reproducibly via Nix.
What’s included

• New flake.nix that packages this repo as repomapper and exposes two executables on PATH: repomap and repomap-mcp (flake.nix:1).
• New flake.lock to pin nixpkgs/flake-utils for reproducible builds (flake.lock:1).
• apps outputs so users can run without installing:
  • nix run .#repomap
  • nix run .#repomap-mcp
• devShell output (nix develop) with Python 3.13 + all runtime dependencies (flake.nix:101).
  Packaging approach
• The derivation installs the Python modules plus the queries/ directory into $out/share/repomapper and wraps python -m repomap / python -m repomap_server with PYTHONPATH pointing at that directory (flake.nix:41).
• This ensures scm.py can still locate Tree-sitter query files via paths relative to the module location.
  Bug fix included
• repomap.py now correctly handles RepoMap.get_repo_map() returning (map_content, file_report) so the CLI works with the current RepoMap API (repomap.py:195).
  How to test
• Build: nix build .#repomapper
• CLI: ./result/bin/repomap --help
• MCP server: nix run .#repomap-mcp (expected to start the FastMCP server / show banner)
• Dev env: nix develop (then run python -m repomap --help)