[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/thread_manager_tests.rs

@@ -432,6 +432,7 @@ async fn start_thread_seeds_extension_data_before_lifecycle_contributors_run() {
         SessionSource::Exec,
         Arc::new(codex_exec_server::EnvironmentManager::default_for_tests()),
         Arc::new(extensions.build()),
+        Arc::new(crate::test_support::EmptyUserInstructionsProvider),
         /*analytics_events_client*/ None,
         thread_store_from_config(&config, /*state_db*/ None),
         /*state_db*/ None,
@@ -481,6 +482,7 @@ async fn resume_and_fork_do_not_restore_thread_environments_from_rollout() {
         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*/ None),
         /*state_db*/ None,
@@ -598,6 +600,7 @@ async fn explicit_installation_id_skips_codex_home_file() {
         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,
         state_db.clone(),
@@ -637,6 +640,7 @@ async fn resume_active_thread_from_rollout_returns_running_thread() {
         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*/ None),
         /*state_db*/ None,
@@ -694,6 +698,7 @@ async fn resume_stopped_thread_from_rollout_spawns_new_thread() {
         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*/ None),
         /*state_db*/ None,
@@ -758,6 +763,7 @@ async fn resume_stopped_thread_from_rollout_preserves_thread_source() {
         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,
         state_db.clone(),
@@ -848,6 +854,7 @@ async fn rollout_path_resume_and_fork_read_history_through_thread_store() {
         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.clone(),
         state_db,
@@ -949,6 +956,7 @@ async fn new_uses_active_provider_for_model_refresh() {
         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*/ None),
         /*state_db*/ None,
@@ -1169,6 +1177,7 @@ async fn interrupted_fork_snapshot_does_not_synthesize_turn_id_for_legacy_histor
         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(),
@@ -1275,6 +1284,7 @@ async fn interrupted_fork_snapshot_preserves_explicit_turn_id() {
         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(),
@@ -1371,6 +1381,7 @@ async fn interrupted_fork_snapshot_uses_persisted_mid_turn_history_without_live_
         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(),