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/turn_context.rs

@@ -1,5 +1,6 @@
 use super::*;
 use crate::SkillLoadOutcome;
+use crate::agents_md::LoadedAgentsMd;
 use crate::config::GhostSnapshotConfig;
 use crate::environment_selection::ResolvedTurnEnvironments;
 use codex_core_skills::HostLoadedSkills;
@@ -551,7 +552,10 @@ impl Session {
             app_server_client_name: session_configuration.app_server_client_name.clone(),
             developer_instructions: session_configuration.developer_instructions.clone(),
             compact_prompt: session_configuration.compact_prompt.clone(),
-            user_instructions: session_configuration.user_instructions.clone(),
+            user_instructions: session_configuration
+                .user_instructions
+                .as_ref()
+                .map(LoadedAgentsMd::text),
             collaboration_mode: session_configuration.collaboration_mode.clone(),
             multi_agent_version,
             personality: session_configuration.personality,