[codex] Document Gemini CLI first-run workaround

[Joseph19820124]

What

• Add a first-run Gemini CLI workaround section to docs/gemini.md.
• Document seeding ~/.gemini/projects.json before the first ACP session in ephemeral containers.
• Note the Kubernetes/Helm limitation that the chart does not yet expose passthrough init/volume fields, with follow-up tracking in chart: expose passthrough fields for Gemini first-run workaround #344.

Why

Gemini-backed openAB deployments can still hit upstream Gemini CLI initialization issues on first boot. This gives users a documented installation-time workaround without changing the image or chart behavior.

Discord Discussion URL: https://discord.com/channels/1488041051187974246/1493631864559243294

Closes #336

[Joseph19820124]

Per the owner reply on #336, this is intentionally kept under docs/* rather than changing the chart or image behavior first. The PR documents the first-run Gemini CLI workaround for ephemeral installs and points out the current Helm/chart limitation around passthrough init/volume fields.\n\nIf you want, I can also trim or reword the workaround section to match the repo’s docs style more closely.

[thepagent]

Closing for now. Don't think it's a good idea to have workaround doc for 3rd party provider in our repo.