quarto add owner/repo fails on repos whose default branch isn't main

[cderv]

quarto add owner/repo (no @ref modifier) only ever tries https://github.com/OWNER/REPO/archive/refs/heads/main.tar.gz. If the repo's default branch is master (or anything else), that request 404s and the resolver gives up with:

Extension not found in local or remote sources

There's no fallback to master and no call to GitHub's API to look up the repo's actual default_branch.

Source: src/extension/extension-host.ts, githubLatestUrlProvider.extensionUrl() (~line 114), invoked via extensionSource()/makeResolvers(). githubBranchUrlProvider can hit an arbitrary branch, but only when the user explicitly writes @branch — it's never tried automatically as a fallback.

Repro:

$ quarto add nickriches/quarto-paper-pile
Extension not found in local or remote sources

$ quarto add nickriches/quarto-paper-pile@master
# installs fine

(nickriches/quarto-paper-pile's default branch is master.) Quarto version: 1.11.1.

Expected: either query the repo's default_branch via the GitHub API before guessing, or fall back to master when main 404s.

Found while reviewing quarto-dev/quarto-web#2149 (a master-default extension submitted to the extension listing).

[mcanouil]

I just fixed the same defect in Quarto Wizard when installing straight from GitHub (the registry json extracts the default branch).