Sync Colab skill auth docs

[givemethatsewon]

Summary

Closes #68.

This syncs the README, session-management design doc, and bundled colab-operator skill with the current authentication and keep-alive behavior. The CLI default is oauth2, and keep-alive now uses the colab.research.google.com tunnel ping rather than the superseded colab.pa.googleapis.com RuntimeService RPC.

Changes

• Update README global --auth default from adc to oauth2.
• Refresh docs/01_session_management.md implementation details and add a log entry for the auth-doc sync.
• Update skills/colab-operator/SKILL.md to describe the OAuth2 remote copy-paste flow, ADC as an explicit option, and current keep-alive failure handling.
• Add a regression test that checks the bundled docs do not drift back to the stale auth/keep-alive wording.

Validation

• uv run pytest tests/test_readme.py
• uv run ruff check tests/test_readme.py
• git diff --check

Note: uv run ruff check . still fails on pre-existing E402 issues in examples/finetune_run.py; this PR does not touch that file.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[givemethatsewon]

Closing this PR because the repository contribution guidelines state external pull requests are not being accepted at this time.