fix(daemon): lock entitlement cache file permissions

[hyperb1iss]

Motivation

• The entitlement cache stored raw JWTs in entitlement.json using tokio::fs::write, which on Unix can produce world-readable files depending on the process umask and risk local token disclosure.
• The change aims to protect the sensitive entitlement JWT on disk by enforcing owner-only permissions when persisting the cache.

Description

• Replace direct tokio::fs::write with a dedicated helper write_entitlement_cache and call it from store_entitlement_response to centralize cache writes.
• On Unix the helper opens the file with OpenOptionsExt::mode(0o600) (via spawn_blocking) to enforce owner-only read/write permissions.
• On non-Unix platforms the helper falls back to tokio::fs::write to preserve existing behavior.
• Extend the unit test cloud_entitlement_cache_round_trips_and_deletes_token to assert the cached file mode is 0600 on Unix.

Testing

• Ran the targeted unit test cloud_entitlement_cache_round_trips_and_deletes_token in the hypercolor-daemon crate; the build/test run was started in the session but did not complete within the time window.
• Added an automated assertion that checks file permissions (0o600) on Unix as part of the existing test to prevent regressions.

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Codex Task

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 9d085827-bf77-4d12-8916-d3ef5760d2d1

📥 Commits

Reviewing files that changed from the base of the PR and between cdffd2b and 30293e4.

📒 Files selected for processing (2)

• crates/hypercolor-daemon/src/cloud_entitlements.rs
• crates/hypercolor-daemon/tests/cloud_api_tests.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/fix-plaintext-caching-of-entitlement-jwt

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