Use public container IP in managed agent prompts

[skulidropek]

Summary

• detect and export a non-loopback container IP in the generated entrypoint
• inject public-access guidance into managed Codex, Claude, and Gemini prompt files
• cover the new prompt text with entrypoint and prepare-files regression tests

Testing

• pnpm install --frozen-lockfile
• pnpm --filter ./packages/lib build
• pnpm --filter ./packages/lib typecheck
• pnpm --filter ./packages/app typecheck
• pnpm --filter ./packages/app exec vitest run tests/docker-git/entrypoint-auth.test.ts
• pnpm --filter ./packages/lib exec vitest run tests/usecases/prepare-files.test.ts

Closes #78.