ci: replace fake julia-actions/setup-julia SHA (824fb972 → 4c0cb0fc)

[hyperpolymath]

Summary

julia-actions/setup-julia@824fb972babf1837cf21c49159bf8a8130f26840 returns HTTP 422 "No commit found" — it is a fake SHA pin, never resolved by GitHub. Every CI run on this repo failed at Unable to resolve action julia-actions/setup-julia@824fb972....

Fix

Replace with the real v3 SHA 4c0cb0fce8556fdb04a90347310e5db8b1f98fb9 (verified via gh api repos/julia-actions/setup-julia/commits/<sha> → 200).

Estate-wide context

One of 8 Julia repos affected. Same fix recipe across all. Caught by the 2026-05-30 estate CI/CD audit.

🤖 Generated with Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 42 issues detected

Severity	Count
🔴 Critical	0
🟠 High	13
🟡 Medium	29

View findings

[
  {
    "reason": "Action perpolymath/standards/.github/workflows/governance-reusable.yml@main\n needs attention",
    "type": "unpinned_action",
    "file": "governance.yml",
    "action": "pin_sha",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in boj-build.yml",
    "type": "missing_timeout_minutes",
    "file": "boj-build.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in casket-pages.yml",
    "type": "missing_timeout_minutes",
    "file": "casket-pages.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in casket-pages.yml",
    "type": "missing_timeout_minutes",
    "file": "casket-pages.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in ci.yml",
    "type": "missing_timeout_minutes",
    "file": "ci.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in codeql.yml",
    "type": "missing_timeout_minutes",
    "file": "codeql.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in governance.yml",
    "type": "missing_timeout_minutes",
    "file": "governance.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in hypatia-scan.yml",
    "type": "missing_timeout_minutes",
    "file": "hypatia-scan.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in instant-sync.yml",
    "type": "missing_timeout_minutes",
    "file": "instant-sync.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in mirror.yml",
    "type": "missing_timeout_minutes",
    "file": "mirror.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  }
]

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