feat: codex project upload

[xgtcode]

Summary

This branch adds two Codex Sandbox handoff capabilities in Studio:

• Allow users to copy a Codex Sandbox Endpoint from the Studio Codex composer, so an existing Sandbox session can be handed off to local tooling.
• Add a Studio-controlled local Codex project upload flow for migrating a local Codex project into a cloud Codex Sandbox.
• Generate a short-lived, one-time project upload authorization code from Studio.
• Let codex-sandbox-upload exchange that authorization code for a persistent Codex Sandbox Endpoint.
• Add a guided Studio dialog with installation instructions and a copyable prompt for local Codex.
• Request a 1-hour authorization TTL and display it as a relative duration instead of exposing the raw UTC expiration timestamp.

Commits

• 0cab088 feat(studio): allow copying sandbox endpoint
• 962f27a feat(studio): add codex project upload handoff

Tests

• uv run pre-commit run --all-files
• uv run pytest tests/cli/test_frontend_sandbox.py -q
• node --test frontend/tests/myAgents.test.mjs frontend/tests/sandboxProjectUpload.test.mjs
• ./frontend/node_modules/.bin/tsc --noEmit -p frontend/tsconfig.json