fix: dead chapter README Colab links

[Jammy2211]

Summary

Part of the Colab link-rot repair (PyAutoLabs/HowToLens#21; phase tracker PyAutoLabs/PyAutoBuild#124). Chapter 1's entry for the removed tutorial_4_complex_models now points at its replacement tutorial_4_why_modeling_is_hard (retitled and re-described to match the tutorial's actual content). Chapter 3's hierarchical-models link now matches the notebook's actual filename tutorial_4_hierachical_models.ipynb — note the repo's filename carries a "hierachical" typo; renaming the script/notebook pair is deliberately out of scope here and can be a tiny follow-up. Both 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

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