fix(gemini): seed registry-shaped projects.json on startup

[Joseph19820124]

This fixes the Gemini first-run crash by seeding ~/.gemini/projects.json with the registry schema expected by Gemini CLI:

{"projects":{}}

Changes:

• add a Gemini-specific entrypoint that seeds the registry file when missing or malformed
• wire Dockerfile.gemini to use that entrypoint
• update docs/gemini.md to reflect the automatic seeding behavior

Validation:

• sh -n docker/gemini/entrypoint.sh
• manual smoke test showed the entrypoint writes {"projects":{}}

Closes #389

[github-actions]

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