feat(studio): add resumable local-to-cloud project handoff

[evanlowe]

Summary

• add the AgentKit Studio plugin and paired handoff workflow for continuing a local coding task in a temporary cloud sandbox
• preserve visible text and image conversation history while restoring the Git-aware project workspace
• upload large project bundles in 64 MiB parts, verify SHA-256 after reassembly, reuse the existing session on retry, and surface upload or restore failures in Studio
• add progress, pairing refresh, completion navigation, and built WebUI assets

Validation

• uv run pytest -q tests/cli/test_frontend_sandbox.py tests/plugins/test_codex_sandbox_upload.py (87 passed)
• npm test (584 passed)
• npm run build
• Ruff 0.11.12 check and format check
• targeted Pyright check (0 errors)
• pre-commit hooks, including gitleaks
• skill and plugin validators
• live 263 MB handoff with four parts, 16 visible messages, one 1730x1676 image, session reuse, workspace restore, and cloud continuation
• reinstall from the Git marketplace and verify the new task discovers plugin version 0.1.1+codex.20260816015419