chore(instructions) Remove unread SessionMeta.instructions field

[dylan-hurd-oai]

Description

• Remove the now-unused instructions field from the session metadata to simplify SessionMeta and stop propagating transient instruction text through the rollout recorder API. This was only saving user_instructions, and was never being read.
• Stop passing user instructions into the rollout writer at session creation so the rollout header only contains canonical session metadata.

Testing

• Ran just fmt which completed successfully.
• Ran just fix -p codex-protocol, just fix -p codex-core, just fix -p codex-app-server, just fix -p codex-tui, and just fix -p codex-tui2 which completed (Clippy fixes applied) as part of verification.
• Ran cargo test -p codex-protocol which passed (28 tests).
• Ran cargo test -p codex-core which showed failures in a small set of tests (not caused by the protocol type change directly): default_client::tests::test_create_client_sets_default_headers, several models_manager::manager::tests::refresh_available_models_*, and shell_snapshot::tests::linux_sh_snapshot_includes_sections (these tests failed in this CI run).
• Ran cargo test -p codex-app-server which reported several failing integration tests (including suite::codex_message_processor_flow::test_codex_jsonrpc_conversation_flow, suite::output_schema::send_user_turn_*, and suite::user_agent::get_user_agent_returns_current_codex_user_agent).
• cargo test -p codex-tui and cargo test -p codex-tui2 were attempted but aborted due to disk space exhaustion (No space left on device).

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Codex Task