docs: add Colab badge and notebook gallery

[mayank-dev-15]

Adds "Open in Colab" badge and notebook gallery to README (issue #435).

• Colab badge in header linking to notebooks/context_rot_live.ipynb
• Notebook Gallery section with the showcase demo
• Zero-install way to try contextweaver interactively

Closes #435.

[dgenio]

Thanks for this — the badge and gallery table render correctly, and the Colab link points to a real notebook (notebooks/context_rot_live.ipynb) on main, so the change itself is solid and low-risk.

One blocker before merge — the issue linkage:

The description says Closes #435, but #435's acceptance criteria are broader than this PR: four new notebooks mirroring the showcase scenarios, an "Open in Colab" badge on docs/showcase.md, a new notebooks/README.md, and a CI execution gate for the notebooks. Merging this would auto-close #435 with most of that work still undone.

Please change Closes #435 → Refs #435 (or Part of #435) so the issue stays open to track the remaining notebooks + CI gate. No code changes needed — I'll approve as soon as the linkage reflects the actual scope.

Nit (non-blocking): the header badge and the gallery-row badge both link to the same single notebook. Fine to keep — just flagging the duplication for when the gallery grows.

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[dgenio]

Heads up — CI is red on the test (3.12) job, and it's caused by this change:

llms drift detected in: llms-full.txt. Run `make llms` and commit the result.

The repo generates llms-full.txt from sources that include README.md, and the gen_llms.py --check gate (runs in the 3.12 matrix cell) fails because the README edit wasn't accompanied by a regenerated artifact. Quick fix:

1. make llms
2. commit the updated llms-full.txt

You can fold this into the same push as the Closes #435 → Refs #435 linkage change from my review. Once both are in, CI should go green and I'll re-review.

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