Gemini image should seed ~/.gemini/projects.json with the registry schema

[Joseph19820124]

Problem

The Gemini image can fail on first ACP startup because Gemini CLI expects ~/.gemini/projects.json to contain a registry object, not an empty JSON object.

Reproduction

1. Deploy openab-gemini with gemini --acp.
2. Start a fresh container/home directory.
3. Gemini CLI exits during startup with a ProjectRegistry/connection closed failure.

Expected

The image should create ~/.gemini/projects.json with:

{"projects":{}}

Actual

Startup can fail if the file is missing or initialized as {}.

Proposed fix

Seed the file in the Gemini image entrypoint before launching openab, and document the behavior for custom images.

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