feat(memory): redact secrets with masking and tests

[Viperdk2020]

Summary

• detect API keys, SSH keys, PEM blocks, and high-entropy strings
• mask secrets and report issues in Redaction
• add tests for redaction behaviour

Testing

• cargo clippy -p codex-memory
• cargo test -p codex-memory

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https://chatgpt.com/codex/tasks/task_e_68b51086ea6c832993c1c72393814679

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