feat(codemode): allow injecting variables into sandbox execution context

[bwebs]

Description

Adds a repeatable --var / -v key=value parameter to lkr code-mode sandbox to define top-level variables inside the Python script execution context.

Closes #43

Summary of Changes

• Added var: list[str] | None option (--var / -v) to the sandbox CLI command in lkr/codemode/main.py.
• Prepended JSON-serialized assignments of passed key=value pairs into the script before Monty execution.
• Updated documentation in codemode.md, lkr/codemode/readme.py, and regenerated lkr.md CLI specs.

[gemini-code-assist]

Code Review

This pull request introduces a new --var (or -v) option to the sandbox command in lkr code-mode, allowing users to inject key-value variables directly into the executed Python script. It also updates the documentation across several files (codemode.md, lkr.md, and lkr/codemode/readme.py) to reflect this new feature, as well as documenting the --version option and the single-file -f/--file option for LookML push/pull commands. Feedback is provided on lkr/codemode/main.py to add validation for the injected variables, ensuring they contain an = character and that the keys are valid Python identifiers, preventing silent failures or syntax errors.