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/BUILD.bazel

@@ -0,0 +1,16 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "instructions",
+    crate_name = "codex_instructions",
+    compile_data = glob(
+        include = ["**"],
+        exclude = [
+            "BUILD.bazel",
+            "Cargo.toml",
+        ],
+        allow_empty = True,
+    ) + [
+        "//codex-rs:node-version.txt",
+    ],
+)