Flatten the `datalex datalex <sub>` CLI nesting

[KKranthi6881]

Problem

The DataLex-spec subcommand tree is currently wired under a nested group, so users type things like:

datalex datalex validate my-project/
datalex datalex dbt sync my-project/

The doubled word is a transitional artifact — it was there to avoid collisions with legacy flat commands (validate, pull, generate, etc.) during the DataLex rollout.

Goal

Register the DataLex subcommands at the top level so users type:

datalex validate my-project/
datalex dbt sync my-project/

Suggested approach

1. Read packages/cli/src/datalex_cli/datalex_cli.py (DataLex subcommand tree) and packages/cli/src/datalex_cli/main.py (legacy flat commands).
2. Enumerate the name collisions between the two sets (validate, diff, info, emit, migrate, …).
3. Pick a resolution per conflict: the DataLex-spec command wins, and the legacy version is renamed to datalex legacy <sub> or removed if clearly unused.
4. Update docs/cli.md, README.md, the tutorial, and any test invocations of datalex datalex ….
5. Add a short datalex --help golden test to pin the flattened tree.

Acceptance criteria

• datalex validate, datalex dbt sync, datalex diff, datalex emit ddl, etc. all work at the top level.
• All existing tests still pass.
• Docs no longer say datalex datalex … anywhere.

Why this is a good first issue

Well-scoped, touches one file mostly, gives you a tour of the CLI surface, and the test suite will catch most regressions. No schema or semantics changes required.

[snakefood3232]

I see the CLI has transitional nesting causing users to type datalex datalex. I'd handle this by merging the command trees in main.py, resolving collisions in favor of DataLex-spec commands and deprecating legacy ones. I can have a PR up in a few hours after updating the code and docs. I've built similar CLI refactors for Python projects using Click.