chore(instructions) Remove unread SessionMeta.instructions field (#9423)

### 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`).

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_696bd8ce632483228d298cf07c7eb41c)

codex-rs/app-server/tests/common/rollout.rs

@@ -57,7 +57,6 @@ pub fn create_fake_rollout(
         cwd: PathBuf::from("/"),
         originator: "codex".to_string(),
         cli_version: "0.0.0".to_string(),
-        instructions: None,
         source: SessionSource::Cli,
         model_provider: model_provider.map(str::to_string),
     };
@@ -135,7 +134,6 @@ pub fn create_fake_rollout_with_text_elements(
         cwd: PathBuf::from("/"),
         originator: "codex".to_string(),
         cli_version: "0.0.0".to_string(),
-        instructions: None,
         source: SessionSource::Cli,
         model_provider: model_provider.map(str::to_string),
     };