Stabilize compact remote parity skills normalization

[starr-openai]

Why

rust-ci-full run 26138722960 failed codex-core::all::suite::compact_remote_parity::remote_compaction_parity_v2_api_key_sends_service_tier_upgrade on macos-aarch64-shard-3 because the parity snapshot compared the full ambient <skills_instructions> body.

That body is environment-derived and can vary independently of the compaction behavior the parity test is trying to compare.

What Changed

• Normalize the contents of a top-level <skills_instructions> block while preserving the block marker itself.
• Add a focused normalization test for that case.

Verification

• Source-grounded against the failing CI log from run 26138722960 / job 76881308783.
• just fmt
• git diff --check
• A post-edit devbox rerun was attempted but blocked before Bazel by local SSH-agent signing failure to dev.