Consolidate team PR review/collaboration guide to docs

[emyller]

We have aligned to a nice pull request reviewing / collaboration guide. We would like to consolidate it into this repository's internal contributing docs — for humans, LLM agents, and whatnot.

Additional scope:

• Point .gemini/styleguide at this new documentation (see this comment for context)

[khvn26]

Is it time for the Flagsmith/agents.md repo?