docs: add ARCHITECTURE.md and link from AGENTS.md

[JoshMock]

I did a full audit of the codebase using ponytail, and all the recommendations it suggested were simplifications of the code that would introduce performance regressions. Fixing those regressions is why the code complexity was introduced in the first place.

So, I've added an ARCHITECTURE.md file, linked from AGENTS.md, that explains each architectural choice, why it exists, and why it should not be removed, unless the improvement can avoid a perf regression.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	0.62s
✅ REPOSITORY	gitleaks	yes		no	no	34.43s
✅ REPOSITORY	git_diff	yes		no	no	0.23s
✅ REPOSITORY	secretlint	yes		no	no	16.76s
✅ REPOSITORY	trivy	yes		no	no	9.91s

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