feat(tooling): replace tox with just

[shrirajpawar4]

🗒️ Description

Replace tox with just as the primary task runner for leanSpec.

This keeps the change scoped to the actual migration surface only:

• add a root Justfile with the common developer and CI commands
• convert tox.ini into a compatibility shim that points old tox env names to the new just recipes
• update CI to invoke just
• update contributor-facing docs and the PR template to use the new command surface

The intent is to simplify local workflows without changing runtime behavior, test semantics, fixture output paths, or package configuration.

Examples of the new interface:

• just check
• just test
• just test-cov-gate
• just fill
• just docs
• just apitest http://localhost:5052

🔗 Related Issues or PRs

Closes #634

Reference:

• feat(ci,tooling): replace tox with just ethereum/execution-specs#2555

✅ Checklist

• Ran local quality checks to avoid unnecessary CI fails:

  just check

• Considered adding appropriate tests for the changes.
• Considered updating the online docs in the ./docs/ directory.

Notes

This PR intentionally does not touch unrelated files such as:

• pyproject.toml
• Docker config
• runtime code
• test logic

tox.ini is kept only as a migration shim so existing contributor habits fail with a clear pointer to the matching just command.

[tcoratger]

@fselmo Feel free to check this one when you have a bit of time :)