Extract codex-instructions crate (#15744)

## Summary
- extract instruction fragment and user-instruction types into
codex-instructions
- update codex-core to consume the new crate

## Testing
- CI

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/instructions/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+edition.workspace = true
+license.workspace = true
+name = "codex-instructions"
+version.workspace = true
+
+[lib]
+doctest = false
+name = "codex_instructions"
+path = "src/lib.rs"
+
+[lints]
+workspace = true
+
+[dependencies]
+codex-protocol = { workspace = true }
+serde = { workspace = true, features = ["derive"] }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }