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-api/src/lib.rs

@@ -26,6 +26,7 @@ pub use codex_config::types::TuiPetAnchor;
 pub use codex_config::types::UriBasedFileOpener;
 pub use codex_core::CodexThread;
 pub use codex_core::ForkSnapshot;
+pub use codex_core::LoadedAgentsMd;
 pub use codex_core::McpManager;
 pub use codex_core::NewThread;
 pub use codex_core::StartThreadOptions;