fix: harden cache-memory pipeline against exec-bit persistence and instruction injection (#1917)

* Initial plan

* fix: harden cache-memory against exec-bit and injection attacks

Agent-Logs-Url: https://github.com/github/gh-aw-firewall/sessions/b0ee2cdb-eb78-4742-995b-6ccf4573fd7e

* fix: tighten injection scan, add quarantine and idempotent update

Agent-Logs-Url: https://github.com/github/gh-aw-firewall/sessions/b0ee2cdb-eb78-4742-995b-6ccf4573fd7e

* fix: address review feedback on cache-memory hardening

- Guard find with directory existence check to avoid failures when
  cache dir is missing (e.g., when prior setup step failed)
- Preserve directory structure in quarantine instead of flattening
  paths, avoiding filename collisions and evidence loss
- Escape head output with sed to prevent GitHub Actions workflow
  command injection from malicious file content
- Apply cache key and restore-keys TTL transformations independently
  so partially updated workflows are correctly repaired

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Landon Cox <landon.cox@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people