[codex] Load user instructions through an injected provider (#27101)

## Why

We want to remove implicit use of `$CODEX_HOME` from `codex-core` and
make embedders responsible for supplying user-level instructions. This
also ensures user instructions load when no primary environment is
selected.

## What changed

Stacked on #27415, which makes `codex exec` surface thread-scoped
runtime warnings.

- Added `UserInstructionsProvider` to `codex-extension-api`, with
absolute source attribution and recoverable loading warnings.
- Added `codex-home` with the filesystem-backed provider for
`AGENTS.override.md` and `AGENTS.md`, preserving precedence, fallback,
trimming, lossy UTF-8 handling, and the existing uncapped global
instruction size.
- Removed global instruction loading from `Config` and require
`ThreadManager` callers to inject a provider.
- Load provider instructions once for each fresh root runtime, including
runtimes without a primary environment. Running sessions retain their
snapshot, while child agents inherit the parent snapshot without
invoking the provider.
- Keep provider instructions separate while loading project `AGENTS.md`,
then assemble the model-visible instructions with the existing ordering,
source attribution, warning, and turn-context behavior.
- Wired the Codex home provider through the CLI, app server, MCP server,
core facade, and thread-manager sample.

## Validation

- `just test -p codex-home -p codex-extension-api`
- `just test -p codex-core agents_md`
- `just test -p codex-core guardian`
- `just test -p codex-app-server
thread_start_without_selected_environment_includes_only_global_instruction_source`
- `just test -p codex-exec warning`
- `just bazel-lock-check`

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

@@ -1,5 +1,3 @@
-use crate::agents_md::DEFAULT_AGENTS_MD_FILENAME;
-use crate::agents_md::LOCAL_AGENTS_MD_FILENAME;
 use crate::config::edit::ConfigEdit;
 use crate::config::edit::ConfigEditsBuilder;
 use crate::config::edit::apply_blocking;
@@ -205,61 +203,6 @@ async fn load_config_normalizes_relative_cwd_override() -> std::io::Result<()> {
     Ok(())
 }
 
-#[tokio::test]
-async fn load_config_loads_global_agents_instructions() -> std::io::Result<()> {
-    let codex_home = tempdir()?;
-    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(),
-        ConfigOverrides::default(),
-        codex_home.abs(),
-    )
-    .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!(
-        user_instructions.sources().collect::<Vec<_>>(),
-        vec![&global_agents_path]
-    );
-    Ok(())
-}
-
-#[tokio::test]
-async fn load_config_prefers_global_agents_override_instructions() -> std::io::Result<()> {
-    let codex_home = tempdir()?;
-    std::fs::write(
-        codex_home.path().join(DEFAULT_AGENTS_MD_FILENAME),
-        "global instructions",
-    )?;
-    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(
-        ConfigToml::default(),
-        ConfigOverrides::default(),
-        codex_home.abs(),
-    )
-    .await?;
-
-    let user_instructions = config
-        .user_instructions
-        .as_ref()
-        .expect("global override instructions expected");
-    assert_eq!(user_instructions.text(), "local override instructions");
-    assert_eq!(
-        user_instructions.sources().collect::<Vec<_>>(),
-        vec![&global_agents_override_path]
-    );
-    Ok(())
-}
-
 #[tokio::test]
 async fn test_toml_parsing() {
     let history_with_persistence = r#"

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

@@ -1,5 +1,3 @@
-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;
@@ -654,9 +652,6 @@ pub struct Config {
     /// Defaults to `false`.
     pub show_raw_agent_reasoning: bool,
 
-    /// User-provided instructions from AGENTS.md.
-    pub user_instructions: Option<LoadedAgentsMd>,
-
     /// Base instructions override.
     pub base_instructions: Option<String>,
 
@@ -2609,12 +2604,6 @@ impl Config {
             .startup_warnings()
             .unwrap_or_default()
             .to_vec();
-        let user_instructions = AgentsMdManager::load_global_instructions(
-            LOCAL_FS.as_ref(),
-            Some(&codex_home),
-            &mut startup_warnings,
-        )
-        .await;
 
         // Destructure ConfigOverrides fully to ensure all overrides are applied.
         let ConfigOverrides {
@@ -3453,7 +3442,6 @@ impl Config {
             approvals_reviewer: constrained_approvals_reviewer.value(),
             enforce_residency: enforce_residency.value,
             notify: cfg.notify,
-            user_instructions,
             base_instructions,
             personality,
             developer_instructions,