feat: accept custom instructions in profiles (#1803)

Allows users to set their experimental_instructions_file in configs.

For example the below enables experimental instructions when running
`codex -p foo`.
```
[profiles.foo]
experimental_instructions_file = "/Users/foo/.codex/prompt.md"
```

# Testing
- ✅ Running against a profile with experimental_instructions_file works.
- ✅ Running against a profile without experimental_instructions_file
works.
- ✅ Running against no profile with experimental_instructions_file
works.
- ✅ Running against no profile without experimental_instructions_file
works.

codex-rs/core/src/config.rs

@@ -480,10 +480,12 @@ impl Config {
         // Load base instructions override from a file if specified. If the
         // path is relative, resolve it against the effective cwd so the
         // behaviour matches other path-like config values.
-        let file_base_instructions = Self::get_base_instructions(
-            cfg.experimental_instructions_file.as_ref(),
-            &resolved_cwd,
-        )?;
+        let experimental_instructions_path = config_profile
+            .experimental_instructions_file
+            .as_ref()
+            .or(cfg.experimental_instructions_file.as_ref());
+        let file_base_instructions =
+            Self::get_base_instructions(experimental_instructions_path, &resolved_cwd)?;
         let base_instructions = base_instructions.or(file_base_instructions);
 
         let config = Self {