docs(codelab): embed local images in generated Colab notebook

[caohy1988]

Summary

Fixes the broken image reference in the checked-in Colab notebook (the non-blocking caveat from #271). The codelab references images by repo-relative path (e.g. images/ca-conversation.png) so claat copies them into img/ — but Colab can't resolve a repo path, so the generated notebook's image was broken.

Change: scripts/generate_colab_from_codelab.py now rewrites local markdown image refs into base64 data URIs during notebook generation (resolved relative to the source markdown's directory). The notebook then renders the image inline in Colab.

• Source codelab markdown is unchanged → claat still uses the relative path and copies the file into img/.
• Remote (http(s)://), data:, and attachment: refs are left untouched.
• A missing local image fails loudly (FileNotFoundError) so a broken ref can't slip through.
• examples/codelab/periodic_materialization/colab_notebook.ipynb regenerated — the CA screenshot is now embedded (1 data URI, 0 relative paths).

Tests

New tests/test_generate_colab_images.py pins the behavior so a future local image can't drift back to a broken Colab ref:

• local image → data:image/png;base64,… (and round-trips to the original bytes)
• remote / data / attachment refs unchanged
• missing local image raises
• build_notebook(..., base_dir=...) embeds; without base_dir it's a no-op (backward compatible)

Test plan

• tests/test_generate_colab_images.py — 5 pass
• Notebook drift --check in sync
• bash autoformat.sh clean (pyink + isort)