[codex] Preserve logical paths during AGENTS.md discovery (#26465)

## Intent

Follow up on #26205 by avoiding unnecessary filesystem canonicalization
during `AGENTS.md` discovery. The configured working directory is
already absolute, and canonicalization incorrectly switches symlinked
workspaces from their logical parent hierarchy to the target's
hierarchy.

## User-facing behavior

For a symlinked working directory such as:

```text
test-root/
|-- logical-repo/
|   |-- AGENTS.md              ("logical parent doc")
|   `-- workspace ------------> physical-repo/workspace/
`-- physical-repo/
    |-- AGENTS.md              ("physical parent doc")
    `-- workspace/
        `-- AGENTS.md          ("workspace doc")
```

Before this change, Codex canonicalized `logical-repo/workspace` to
`physical-repo/workspace` before discovery. It therefore loaded
`physical-repo/AGENTS.md` and `physical-repo/workspace/AGENTS.md`,
ignoring the instructions from the repository through which the user
entered the workspace.

After this change, ancestor discovery walks the configured logical path,
so Codex loads `logical-repo/AGENTS.md`. Opening
`logical-repo/workspace/AGENTS.md` still follows the symlink through the
host filesystem, so the workspace document is also loaded.
`physical-repo/AGENTS.md` is not loaded.

## Implementation

Use the logical absolute working directory when discovering project
instructions and reporting instruction sources. Filesystem reads still
follow the working-directory symlink, so an `AGENTS.md` in the target
workspace continues to load while ancestor discovery uses the symlink's
parents.

## Validation

Added integration coverage proving that discovery loads the logical
parent's instructions and the target workspace's instructions, but not
the target parent's instructions.

codex-rs/core/src/agents_md.rs

@@ -206,10 +206,7 @@ impl<'a> AgentsMdManager<'a> {
             return Ok(Vec::new());
         }
 
-        let mut dir = self.config.cwd.clone();
-        if let Ok(canonical_dir) = fs.canonicalize(&dir, /*sandbox*/ None).await {
-            dir = canonical_dir;
-        }
+        let dir = self.config.cwd.clone();
 
         let mut merged = TomlValue::Table(toml::map::Map::new());
         for layer in self.config.config_layer_stack.get_layers(