ci(#983): add pre-commit hook to verify URL #sha256= integrity pins

[fullsend-ai-coder]

Summary

• Add hack/verify-sha256-pins script that scans harness/*.yaml and .fullsend/config.yaml for URL+#sha256= integrity fragments, fetches each URL, and verifies the pinned hash matches the actual content. Field-agnostic scanning covers all current and future YAML fields. Container image digests (@sha256:) and placeholder/template values are correctly skipped.
• Wire the script as a local pre-commit hook (verify-sha256-pins) in .pre-commit-config.yaml, following the lint-agent-docs pattern so the lint.yml workflow runs it automatically.
• Add comprehensive test suite (hack/verify-sha256-pins-test.sh) with 11 test cases and register it in the Makefile script-test target.

Testing

• All 11 test cases pass: correct hash, wrong hash, local paths, image digests, field-agnostic multi-field scanning, .fullsend/config.yaml inclusion, fetch failure, placeholder skip, mismatch output details, mixed valid/invalid, and no-YAML-files edge case.
• shellcheck passes on both scripts.
• gitlint passes on commit message.

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Closes #983

Post-script verification

• Branch is not main/master (agent/983-verify-sha256-pins)
• Secret scan passed (gitleaks — 299f264ab55ffda8bd297642d787f751d6984789..HEAD)
• PR body secret scan passed (gitleaks — no-git)

[fullsend-ai-review]

🤖 Finished Review · ✅ Success · Started 10:54 AM UTC · Completed 11:12 AM UTC

Commit: a1c332f · View workflow run →

Runtime: claude · Model: opus → claude-opus-4-6 · Effort: high · Cost: $4.53

[fullsend-ai-review]

Review

Findings

Low

• [edge-case] hack/verify-sha256-pins:81 — Case-sensitive hash comparison may produce false mismatches. The extraction regex accepts uppercase hex ([0-9a-fA-F]+), but sha256sum always outputs lowercase. If a YAML file pins a hash using uppercase characters, the comparison would report a mismatch even though the hashes are semantically identical.
  Remediation: Normalize expected to lowercase before comparison, or restrict the extraction regex to lowercase hex only ([0-9a-f]+).

• [command-injection] hack/verify-sha256-pins:76 — VERIFY_FETCH is expanded unquoted ($FETCH_CMD "$url") to allow word splitting for multi-word commands like curl -sfL. A maliciously crafted VERIFY_FETCH value could inject arbitrary arguments, but the variable is only settable by the person or CI environment invoking the script — not by PR content.

• [ssrf] hack/verify-sha256-pins:76 — The script fetches all URLs with #sha256= fragments from harness YAML files. A malicious PR could add a URL targeting internal infrastructure. Mitigated: fetched content is only used for sha256 comparison (not exfiltrated), curl -sfL suppresses output on failure, and URLs in YAML files go through code review.