docs(proposals): Deprecate PEP 508 Direct References (req.url)

[smoparth]

Propose deprecating the package @ git+https://... syntax in favor of the declarative source provider config. The feature requires 19 special-case branches across 6 files and every use case is covered by the new git-clone providers (pypi-git, github-tag-git, gitlab-tag-git, versionmap-git).

Pull Request Description

What

Why

• PR follows CONTRIBUTING.md guidelines

[coderabbitai]

📝 Walkthrough

Walkthrough

This change adds a new architecture decision record proposing deprecation and eventual removal of PEP 508 direct reference URL (req.url) support as a top-level bootstrap input. It defines scope limits, maps req.url use cases to declarative source provider configuration, and outlines a three-phase plan: emit deprecation warnings, provide migration guidance, and remove special-case code paths. No code or public API changes are included.

Estimated code review effort: 1 (Trivial) | ~3 minutes

Sequence Diagram(s)

No diagram generated, as this change is a documentation-only addition with no code flow to depict.

Related issues: None mentioned in the provided context.

Related PRs: None mentioned in the provided context.

Suggested labels: documentation, proposal

Suggested reviewers: None specified.

🐰 A note is penned, no code to see,
Just plans for URLs' future decree,
"req.url", we bid adieu,
Sources will guide us through,
A rabbit's scroll of things to be.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly states the main change: deprecating PEP 508 direct references (req.url).
Description check	✅ Passed	The description is directly about deprecating req.url in favor of declarative source providers.

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[rd4398]

This aligns with what we discussed earlier today. @dhellmann originally added this feature and we should wait for him to take a look and see what he thinks.

[tiran]

Nit-pick: We don't use the term "ADR" in upstream Fromager.