Preserve modes for trusted read-only sessions

[liaowang11]

Summary

• match approval presets by sandbox variant and network semantics instead of full sandbox struct equality
• keep modes available for trusted default, trusted read-only, trusted workspace-write, full-access, external-sandbox, and untrusted fallback cases
• add isolated regression coverage that loads config against temporary Git worktrees so trust-level behavior is exercised under real project roots

Testing

• Added thread::tests::test_load_keeps_modes_for_untrusted_session.
• Added thread::tests::test_load_keeps_modes_for_trusted_default_session.
• Kept and validated thread::tests::test_load_keeps_modes_for_trusted_read_only_policy.
• Added thread::tests::test_load_keeps_modes_for_trusted_workspace_write_policy.
• Added thread::tests::test_load_keeps_modes_for_full_access_policy.
• Added thread::tests::test_load_keeps_modes_for_external_sandbox_with_network_access.
• Results: local Rust test suite passed with 27 passed, 0 failed for src/lib.rs and 0 passed, 0 failed for src/main.rs.

Closes #201

[benbrandt]

thanks! I think solved in #200