Route AGENTS.md loading through environment filesystems (#26205)

## Why

Workspace-specific `AGENTS.md` loading needs to use the selected
environment filesystem so remote workspaces and child agents read
instructions from their actual environment instead of the host
filesystem. The app-server should report the same instruction sources
the initialized thread actually loaded, rather than independently
rescanning configuration and filesystem state.

## What changed

- Introduce `LoadedAgentsMd` to retain ordered user, project, and
internal instructions with their provenance.
- Load and canonicalize workspace `AGENTS.md` paths through the primary
`EnvironmentManager` environment, then render the loaded instructions
when constructing turn context.
- Expose cached loaded instruction sources from initialized threads and
use them for app-server start, resume, and fork responses.
- Preserve global `CODEX_HOME` loading and separator behavior while
excluding empty project files that did not supply model-visible
instructions.
- Add integration coverage for CLI injection, selected-environment
provenance and rendering, empty environment selection, and cached
sources on loaded-thread resume.

## Validation

- `just test -p codex-core agents_md`
- `just test -p codex-core
selected_environment_sources_match_model_visible_instructions`
- `just test -p codex-exec agents_md`
- `just test -p codex-app-server instruction_sources`
- `just test -p codex-app-server --status-level fail`

codex-rs/core/src/config/config_tests.rs

@@ -208,10 +208,8 @@ async fn load_config_normalizes_relative_cwd_override() -> std::io::Result<()> {
 #[tokio::test]
 async fn load_config_loads_global_agents_instructions() -> std::io::Result<()> {
     let codex_home = tempdir()?;
-    std::fs::write(
-        codex_home.path().join(DEFAULT_AGENTS_MD_FILENAME),
-        "\n  global instructions  \n",
-    )?;
+    let global_agents_path = codex_home.abs().join(DEFAULT_AGENTS_MD_FILENAME);
+    std::fs::write(&global_agents_path, "\n  global instructions  \n")?;
 
     let mut config = Config::load_from_base_config_with_overrides(
         ConfigToml::default(),
@@ -221,9 +219,14 @@ async fn load_config_loads_global_agents_instructions() -> std::io::Result<()> {
     .await?;
     let _ = config.features.enable(Feature::MemoryTool);
 
+    let user_instructions = config
+        .user_instructions
+        .as_ref()
+        .expect("global instructions expected");
+    assert_eq!(user_instructions.text(), "global instructions");
     assert_eq!(
-        config.user_instructions.as_deref(),
-        Some("global instructions")
+        user_instructions.sources().collect::<Vec<_>>(),
+        vec![&global_agents_path]
     );
     Ok(())
 }
@@ -235,7 +238,7 @@ async fn load_config_prefers_global_agents_override_instructions() -> std::io::R
         codex_home.path().join(DEFAULT_AGENTS_MD_FILENAME),
         "global instructions",
     )?;
-    let global_agents_override_path = codex_home.path().join(LOCAL_AGENTS_MD_FILENAME);
+    let global_agents_override_path = codex_home.abs().join(LOCAL_AGENTS_MD_FILENAME);
     std::fs::write(&global_agents_override_path, "local override instructions")?;
 
     let config = Config::load_from_base_config_with_overrides(
@@ -245,9 +248,14 @@ async fn load_config_prefers_global_agents_override_instructions() -> std::io::R
     )
     .await?;
 
+    let user_instructions = config
+        .user_instructions
+        .as_ref()
+        .expect("global override instructions expected");
+    assert_eq!(user_instructions.text(), "local override instructions");
     assert_eq!(
-        config.user_instructions.as_deref(),
-        Some("local override instructions")
+        user_instructions.sources().collect::<Vec<_>>(),
+        vec![&global_agents_override_path]
     );
     Ok(())
 }

codex-rs/core/src/config/mod.rs

@@ -1,4 +1,5 @@
 use crate::agents_md::AgentsMdManager;
+pub use crate::agents_md::LoadedAgentsMd;
 use crate::config::edit::ConfigEdit;
 use crate::config::edit::ConfigEditsBuilder;
 use crate::path_utils::normalize_for_native_workdir;
@@ -645,7 +646,7 @@ pub struct Config {
     pub show_raw_agent_reasoning: bool,
 
     /// User-provided instructions from AGENTS.md.
-    pub user_instructions: Option<String>,
+    pub user_instructions: Option<LoadedAgentsMd>,
 
     /// Base instructions override.
     pub base_instructions: Option<String>,
@@ -2605,8 +2606,7 @@ impl Config {
             Some(&codex_home),
             &mut startup_warnings,
         )
-        .await
-        .map(|loaded| loaded.contents);
+        .await;
 
         // Destructure ConfigOverrides fully to ensure all overrides are applied.
         let ConfigOverrides {