projects: show badge URL in staff debug modal

[ericholscher]

HS: Had a support request for the badge url, and didn't have a way to even give it to them manually.

Summary

The ext-theme dashboard currently has no UI for retrieving a project's badge URL (the old dashboard rendered the embed snippets next to the badge image; that wasn't ported. For private projects, the URL is non-guessable — it requires a token computed from SECRET_KEY + slug server-side — so users have no way to construct it themselves.

As a short-term measure so we can hand the URL to users who ask, this exposes the badge URL for the project's default version inside the staff-only project debug modal in partials/header.html.

The URL is generated via a new get_project_badge_url template tag in ext_theme_tags.py that wraps the existing ProjectBadgeView.get_badge_url classmethod. For private versions this already appends the &token= query param.

Test plan

• Visit a project's home page as a staff user → click Debug → confirm the badge URL is rendered and clicking/copying it loads the right SVG
• Repeat for a private-version project → confirm the URL contains &token= and resolves the correct badge state
• Confirm the modal still renders for non-staff admins without the Debug button being visible (it's gated by request.user.is_staff)

Notes

• The first commit is unrelated prettier reformatting of AGENTS.md — pre-existing violations from the recent sync from readthedocs/common that the pre-commit prettier hook required. Happy to split this out if preferred.
• This is a placeholder until we port the full badge-embed UI (RST/Markdown/HTML snippets via core/badge_markup.html) back to the ext-theme project home page.

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