[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/mcp-server/src/message_processor.rs

@@ -7,6 +7,7 @@ use codex_core::ThreadManager;
 use codex_core::config::Config;
 use codex_exec_server::EnvironmentManager;
 use codex_extension_api::empty_extension_registry;
+use codex_home::CodexHomeUserInstructionsProvider;
 use codex_login::AuthManager;
 use codex_login::default_client::USER_AGENT_SUFFIX;
 use codex_login::default_client::get_codex_user_agent;
@@ -62,12 +63,16 @@ impl MessageProcessor {
             /*enable_codex_api_key_env*/ false,
         )
         .await;
+        let user_instructions_provider = Arc::new(CodexHomeUserInstructionsProvider::new(
+            config.codex_home.clone(),
+        ));
         let thread_manager = Arc::new(ThreadManager::new(
             config.as_ref(),
             auth_manager,
             SessionSource::Mcp,
             environment_manager,
             empty_extension_registry(),
+            user_instructions_provider,
             /*analytics_events_client*/ None,
             codex_core::thread_store_from_config(config.as_ref(), state_db.clone()),
             state_db.clone(),