feat: change default user from 'agent' to 'fulling' with updated workdir (#89)

[Che-Zhu]

Summary

• Change default user from agent to fulling across all sandbox configurations
• Update working directory from /home/agent to /home/fulling
• Update volume mount paths and PVC names to reflect new user
• Modify shell configuration to work with new home directory
• Maintain same UID/GID (1001:1001) for compatibility

Changes

• Dockerfile: User creation, PATH, and WORKDIR updated
• Sandbox Manager: Volume mounts, PVC names, and init script paths updated
• StatefulSet YAML: Volume claim templates and mount paths updated
• .bashrc: Path functions updated for new home directory

Test plan

• Build new runtime image with updated Dockerfile
• Deploy sandbox environment and verify user is fulling
• Confirm working directory is /home/fulling
• Verify volume mounts are correctly pointing to /home/fulling
• Test terminal access and basic operations
• Verify Next.js project template creation works correctly

Closes #89

🤖 Generated with Claude Code

[github-actions]

✅ PR Check Results: Passed

Build Checks

Check	Status
Lint & Build	✅ Passed
Docker Build	✅ Passed

✨ Great work!

All checks passed successfully. Your PR is ready for review.

Details:

• ✅ Code quality verified (linting passed)
• ✅ Build successful
• ✅ Docker image build verified (linux/amd64)
  Commit: 33c5f30ad0ab977b1c429f96416bba00034a2a36
  Branch: change_user_workdir

🔗 View Details:

• Lint & Build
• Docker Build