docs: remove outdated Gemini CLI support references

[siddqamar]

Problem

Google has discontinued Gemini CLI support, but our documentation still lists Gemini CLI as a supported Graphify platform.

This makes the docs outdated and may send users toward install commands or hook setup paths that no longer work.

Outdated docs include

• Gemini CLI in supported platform lists
• graphify install --platform gemini
• graphify gemini install
• Gemini CLI hook / BeforeTool references
• GEMINI.md setup references

Expected update

• Remove Gemini CLI from the current documentation.
• Remove stale Gemini CLI install and hook instructions.
• Keep the cleanup scoped to documentation only.

Notes

This is a documentation cleanup only. No runtime behavior changes are required.