CI: quarantine upstream workflows not needed on fork

[sydneyrenee]

Summary

• Moved 9 inherited matplotlib workflows out of .github/workflows/ into github-workflows-quarantine/ to stop them from running on the fork
• Added github-workflows-quarantine/README.md documenting provenance, why each was quarantined, and how to restore them
• Deleted all accumulated run history for the quarantined workflows

Quarantined workflows

File	Trigger	Problem
nightlies.yml	cron (daily)	Tries to upload wheels to Anaconda Cloud with missing ANACONDA_TOKEN — would publish to scientific-python-nightly-wheels if the secret existed
stale.yml	cron (3x/week)	Labels fork PRs as inactive on upstream's schedule
stale-tidy.yml	cron (3x/week)	Closes fork issues as inactive on upstream's schedule
cibuildwheel.yml	push to main	Builds release wheels on every push — expensive CI minutes for a dev fork
cygwin.yml	push to main	Runs Cygwin build/test matrix — irrelevant for MLX porting work
circleci.yml	status events	Requires CIRCLECI_TOKEN secret that doesn't exist on the fork
good-first-issue.yml	issue labeled	Posts upstream community onboarding text
pr_welcome.yml	PR opened	Posts upstream contributor greeting
labeler.yml	PR opened	Auto-labels PRs using upstream's label config

Kept active

tests.yml, linting.yml, codeql-analysis.yml, mypy-stubtest.yml, clean_pr.yml, do_not_merge.yml, conflictcheck.yml

Test plan

• Verify no scheduled workflows fire after merge
• Verify remaining CI still runs on PRs

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