feat: add 4 new error entries (silent-failures, runner-environment x2, known-unsolved)

[htekdev]

New Error Entries — Strategy 4 Round 240 (Runner Image Updates + GitHub Issues Mining)

Merged PRs first: #388 (re-476, re-477, re-478) and #389 (sf-215, ca-138, ca-139, re-479) — both CI green, both squash-merged. Live total: 1390.

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Entries Added

ID	Title	Category	Source
sf-216	Secret containing $ character silently expands in double-quoted shell string	silent-failures	actions/runner#4500
re-480	envsubst: command not found on ARC and ubuntu-slim — gettext not pre-installed	runner-environment	sigstore/cosign-installer#222
re-481	ubuntu-slim ships git 2.43.0 instead of documented 2.52+	runner-environment	actions/runner-images#14014
ku-131	setup-node mirror input is only a fallback — node pulled from github.com when version is in manifest	known-unsolved	actions/setup-node#1412

Dedup Summary

Checked and confirmed NOT already present before adding:

• rustup-init +stable argv rejected → already covered (macos-15-arm64-homebrew-rustup-init-argv-rejected.yml)
• Temurin LTS build suffix version mismatch → already covered (setup-java-temurin-lts-suffix-version-file-mismatch.yml)
• bpftool not found kernel mismatch → already covered (ubuntu-2404-bpftool-azure-kernel-no-matching-package.yml)
• JBR invalid version parsing → already covered (setup-java-jbr-invalid-version-dotted-build-metadata.yml)
• checkout EU slow / timeout → already covered (checkout-eu-runner-timeout-regional-degradation.yml)
• cache save pull_request_target poisoning → already covered
• checkout v6.0.3 hash-algorithm rate limit → already covered
• cache BusyBox tar -P alpine → already covered (ca-133)
• AAD ghost-busy runner → already covered
• govulncheck broken → already covered

New entries total after this PR: 1394