[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/thread-manager-sample/src/main.rs

@@ -16,6 +16,7 @@ use codex_core_api::AskForApproval;
 use codex_core_api::AuthCredentialsStoreMode;
 use codex_core_api::AuthManager;
 use codex_core_api::AutoCompactTokenLimitScope;
+use codex_core_api::CodexHomeUserInstructionsProvider;
 use codex_core_api::CodexThread;
 use codex_core_api::Config;
 use codex_core_api::ConfigLayerStack;
@@ -120,12 +121,16 @@ async fn run_main(arg0_paths: Arg0DispatchPaths) -> anyhow::Result<()> {
             .await?,
     );
     let installation_id = resolve_installation_id(&config.codex_home).await?;
+    let user_instructions_provider = Arc::new(CodexHomeUserInstructionsProvider::new(
+        config.codex_home.clone(),
+    ));
     let thread_manager = ThreadManager::new(
         &config,
         auth_manager,
         SessionSource::Exec,
         environment_manager,
         empty_extension_registry(),
+        user_instructions_provider,
         /*analytics_events_client*/ None,
         Arc::clone(&thread_store),
         state_db,
@@ -184,7 +189,6 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         enforce_residency: Constrained::allow_any(/*initial_value*/ None),
         hide_agent_reasoning: false,
         show_raw_agent_reasoning: false,
-        user_instructions: None,
         base_instructions: None,
         developer_instructions: None,
         guardian_policy_config: None,