feat(common): Make python-common installable

[LucienShui]

Problem

python-common is currently consumed as a source submodule and has no installable package metadata. Its project_root lookup also walks forever when the package is installed under site-packages.

Changes

• Add setuptools package metadata and runtime dependencies.
• Package the existing repository root as the common import package without moving files.
• Resolve the consumer project root from the importing caller when the package is installed.

Validation

• uv build
• Installed the wheel into a simulated site-packages directory.
• Verified project_root resolves to the consuming project's directory and excludes site-packages.