Add a root `justfile` for easier development workflows

Motivation

Sapling contains several components with different build and test workflows For example, building the CLI currently involves running make oss from eden/scm, while other components use CMake, Cargo, Yarn, or getdeps.py.

Would the project be open to adding a root-level justfile that provides a consistent entry point for common contributor tasks?

Proposed approach

The justfile would wrap the existing build systems rather than replace CMake. Possible recipes could include:

• just build-cli
• just test-cli
• just build-edenfs
• just test
• just lint
• just build-isl

The existing commands would remain usable directly, and CMake would continue to manage native builds.

Benefits

• Easier command discovery through just --list
• A consistent interface from the repository root
• Less need for contributors to remember component-specific working directories
• No immediate migration or disruption to existing CI workflows

Questions

• Would maintainers find a root-level task runner useful?
• Which workflows should an initial justfile expose?
• Would adding just as a development dependency be acceptable?