fix: add grok to pi models configuration

[MatrixNeoKozak]

What

Add grok provider to models.json pi configuration to support grok CLI usage as requested in issue #105.

Why

This change resolves the target issue or improvement.

How to test

Verify that the project builds/runs correctly and the specific bug/improvement is addressed.

Fixes #105

[michael-denyer]

Thanks for the PR. I'm closing it because the new table can't be reached and its model ids aren't in Pi's catalog.

• pstack picks the table by the provider of the session's model. Pi signs Grok in as xai, not grok, so an xAI session still falls back to anthropic.
• Pi's xai catalog lists grok-4.3 to grok-4.7. The -xhigh-fast and -medium-fast suffixes are Cursor slugs; Pi sends reasoning effort separately. CI's catalog check fails on all four entries.

Pi users on xAI can already map the aliases with one line in ~/.pi/agent/pstack-models.md:

pi models: opus=xai/grok-4.7, fable=xai/grok-4.7, sonnet=xai/grok-4.6, haiku=xai/grok-4.6

#105 asks for Grok to run as a Claude Code subworker through the Grok CLI, which this PR doesn't touch, so it stays open.