[sandboxing] Remove Darwin user cache write from Seatbelt network policy

[evawong-oai]

Summary

1. Removes the broad DARWIN_USER_CACHE_DIR write rule from the macOS Seatbelt network policy.
2. Removes the now unused policy parameter plumbing for that cache path.
3. Adds sandboxing coverage that keeps com.apple.trustd.agent for TLS while rejecting the cache write rule.

Why

This closes the exact cache poisoning boundary. The earlier gh TLS issue is now covered by trustd access, so the cache write is no longer needed.

Validation

1. Rust formatting passed.
2. The sandboxing crate tests passed.
3. Local macOS Seatbelt repro with patched policy passed. gh api returned 21442 without the cache write rule.

[viyatb-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

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