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/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 {