memories: exclude AGENTS and skills from stage1 input

[andi-oai]

Why/Context/Summary

• Exclude injected AGENTS.md instructions and standalone skill payloads from memory stage 1 inputs so memory generation focuses on conversation content instead of prompt scaffolding.
• Strip only the AGENTS fragment from mixed contextual user messages during stage-1 serialization, which preserves environment context in the same message.
• Keep subagent notifications in the memory input, and add focused unit coverage for the fragment classifier, rollout policy, and stage-1 serialization path.

Test plan

• just fmt
• cargo test -p codex-core --lib contextual_user_message
• cargo test -p codex-core --lib rollout::policy
• cargo test -p codex-core --lib memories::phase1