🔒 [security] encrypt token cache and configuration at rest

[cmuench]

This PR addresses a security vulnerability where authentication tokens and secrets were stored in plaintext JSON files.

🎯 What: Introduced encryption at rest for config.json and token-cache.json.
⚠️ Risk: Storing secrets in plaintext on disk exposes them to any local user or process with read access, which is especially risky on multi-tenant developer workstations.
🛡️ Solution:

• Implemented encryption using Node.js native crypto module with aes-256-ctr.
• Derived a semi-stable encryption key from os.hostname() and os.userInfo().username.
• Updated saveConfig, loadConfig, saveTokenCache, and loadTokenCache in lib/config.js to handle encrypted data.
• Added a fallback mechanism to support existing plaintext configuration files.
• Updated tests/config.test.js to account for the new encryption layer.
• Added tests/token_encryption_repro.test.js to verify encryption works as expected.

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PR created automatically by Jules for task 7106440122432922954 started by @cmuench

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