[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/tools/handlers/multi_agents_tests.rs

@@ -1,5 +1,4 @@
 use super::*;
-use crate::LoadedAgentsMd;
 use crate::ThreadManager;
 use crate::config::AgentRoleConfig;
 use crate::config::DEFAULT_AGENT_MAX_DEPTH;
@@ -4241,6 +4240,7 @@ async fn tool_handlers_cascade_close_and_resume_and_keep_explicitly_closed_subtr
         SessionSource::Exec,
         Arc::new(codex_exec_server::EnvironmentManager::default_for_tests()),
         empty_extension_registry(),
+        Arc::new(crate::test_support::EmptyUserInstructionsProvider),
         /*analytics_events_client*/ None,
         thread_store_from_config(&config, state_db.clone()),
         state_db.clone(),
@@ -4520,25 +4520,6 @@ async fn build_agent_spawn_config_uses_turn_context_values() {
     assert_eq!(config, expected);
 }
 
-#[tokio::test]
-async fn build_agent_spawn_config_preserves_base_user_instructions() {
-    let (_session, mut turn) = make_session_and_context().await;
-    let mut base_config = (*turn.config).clone();
-    base_config.user_instructions = Some(LoadedAgentsMd::new_user(
-        "base-user".to_string(),
-        base_config.codex_home.join("AGENTS.md"),
-    ));
-    turn.user_instructions = Some("resolved-user".to_string());
-    turn.config = Arc::new(base_config.clone());
-    let base_instructions = BaseInstructions {
-        text: "base".to_string(),
-    };
-
-    let config = build_agent_spawn_config(&base_instructions, &turn).expect("spawn config");
-
-    assert_eq!(config.user_instructions, base_config.user_instructions);
-}
-
 #[tokio::test]
 async fn build_agent_resume_config_clears_base_instructions() {
     let (_session, mut turn) = make_session_and_context().await;