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/session/session.rs

@@ -1,5 +1,6 @@
 use super::input_queue::InputQueue;
 use super::*;
+use crate::agents_md::LoadedAgentsMd;
 use crate::config::ConstraintError;
 use crate::goals::GoalRuntimeState;
 use crate::skills::SkillError;
@@ -54,8 +55,9 @@ pub(crate) struct SessionConfiguration {
     /// Developer instructions that supplement the base instructions.
     pub(super) developer_instructions: Option<String>,
 
-    /// Model instructions that are appended to the base instructions.
-    pub(super) user_instructions: Option<String>,
+    /// Model instructions that are appended to the base instructions and the
+    /// files that supplied them.
+    pub(super) user_instructions: Option<LoadedAgentsMd>,
 
     /// Personality preference for the model.
     pub(super) personality: Option<Personality>,