chart: expose passthrough fields for Gemini first-run workaround

[Joseph19820124]

Context

PR #343 documents the Gemini CLI first-run workaround in docs/gemini.md.

Follow-up

The current chart still does not expose passthrough fields for agent-specific init containers and volumes.

Requested fields

• extraInitContainers
• extraVolumes
• extraVolumeMounts

Why

This keeps the workaround out of the image/chart by default, but gives Helm users a supported way to seed ~/.gemini before the first ACP session.

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