[Foundation] Create installable package, source layout, and reproducible toolchain

[Dyu20705]

Problem statement

RunSift needs an installable, reproducible Python project foundation before feature work begins. A clean checkout must produce the same CLI, checks, and package artifacts locally and in CI.

Scope

• create pyproject.toml with explicit build backend and project metadata;
• use a src/runsift/ package layout and tests/ layout;
• expose the runsift console entry point;
• define supported Python versions and dependency groups;
• configure formatting, linting, typing, and test commands;
• build wheel and source distribution artifacts;
• document one clean setup flow for contributors;
• keep runtime dependencies minimal and pinned by declared compatibility ranges.

Checklist

• Add package and build configuration
• Add src/runsift/ and test skeletons
• Add console-script entry point
• Define supported Python versions
• Define runtime and development dependency groups
• Add build, lint, type-check, and test commands
• Build wheel and sdist in CI
• Document clean-checkout setup

Acceptance criteria

• A clean checkout can install RunSift and run runsift --help
• Tests and static checks run through documented commands
• Wheel and sdist build successfully
• Import boundaries do not depend on the repository working directory
• Runtime dependencies are justified and minimal
• Local and CI commands are equivalent

Dependencies

• Implements the repository foundation described in docs/roadmap.md
• Parent epic: [Epic] CLI and Project Foundation #2