Add sanitize_obj helper for redacting sensitive data

[binrogithub]

Motivation

• Provide a reusable utility to remove or mask sensitive values (tokens, secrets, certs, kubeconfigs) from arbitrary data structures.
• Handle very large strings and binary blobs by replacing them with a deterministic hashed redaction marker while preserving data shape for logging and diagnostics.

Description

• Add hc_agent/core/sanitize.py implementing sanitize_obj() and helpers for key detection and redaction.
• Detect sensitive keys using case-insensitive substring matching against ("token","secret","cert","kubeconfig").
• Replace large strings and bytes with <redacted sha256=... len=...> markers computed via hashlib.sha256, while leaving small values intact.
• Recursively sanitize nested structures and common container types (dict, list, tuple, set) to preserve structure.

Testing

• No automated tests were run for this change.

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Codex Task