Document the three installation methods:
- Debian/Ubuntu .deb package with shell completions
- Statically-linked binary for any Linux system
- Nix flake and Home Manager module
Document the new todo command features:
- Numbered task display
- `streamd todo N edit` for editing tasks
- `streamd todo N done` for marking tasks done
- `--show-future` flag for including future tasks
- Add R21 specification for todo command behavior
- Rename src/streamer/ to src/streamd/
- Update all internal imports
- Update pyproject.toml project name and entry point
- Update README branding (Streamer -> Strea.md)
- Switch from pyright to basedpyright
- Bump requires-python to >=3.13
