fix: repoint chapter README Colab URLs at HowToGalaxy + align entries with actual tutorials

[Jammy2211]

Summary

Part of the Colab link-rot repair (PyAutoLabs/HowToLens#21; phase tracker PyAutoLabs/PyAutoBuild#124). All 28 chapter README Colab links pointed at autogalaxy_workspace, where the chapter notebooks no longer live — dead links that the release bumper has been dutifully re-tagging every release. They now point at this repo at the current tag. The README entries had also been copied from HowToLens with lens-specific tutorials that don't exist here; they now match the actual notebook set: chapter 3 lists tutorial_3_x2_galaxies (the copied lens entries 3–6 removed), chapter 4 ends at tutorial_5_model_fit (borders / lens-modeling / adaptive-pixelization entries removed). The 28 grandfathered Colab entries leave .url_check_allowlist.txt.

Scripts Changed

None — chapter/root README link fixes and .url_check_allowlist.txt purges only; no scripts or notebooks execute differently.

Validation checklist (--auto run — plan was not pre-approved)

• Effective level: safe (header: safe, cap: feature/maintenance → safe at medium difficulty)
• Plan: on the issue (fix: Colab link rot — pin/repoint chapter README URLs, purge allowlists (phase 2) HowToLens#21), written at launch, unmodified since
• Gate: tests n/a (no test dirs — documentation/tutorial repos) · smoke n/a (README/allowlist-only diff, no script surface) · review CLEAN · Heart YELLOW within launch-acknowledged reason set (5 pre-existing reasons, no new)
• Link-target verification: every Colab URL in the four repos machine-checked — canonical owner, current tag 2026.7.6.649, target notebook exists; Heart's hardened offline guard (url_check.sh, incl. the new phase-1 patterns) exits clean on all four repos
• Human: plan sound in hindsight?
• Human: diff matches plan (no scope creep)?
• Human: merge, amend, or reject — then log the outcome

🤖 Generated with Claude Code