[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/cli/src/main.rs

@@ -41,6 +41,7 @@ use std::collections::HashSet;
 use std::io::IsTerminal;
 use std::io::Write;
 use std::path::PathBuf;
+use std::sync::Arc;
 use supports_color::Stream;
 
 #[cfg(any(target_os = "macos", target_os = "windows"))]
@@ -75,6 +76,7 @@ use codex_core::config::resolve_profile_v2_config_path;
 use codex_features::FEATURES;
 use codex_features::Stage;
 use codex_features::is_known_feature_key;
+use codex_home::CodexHomeUserInstructionsProvider;
 use codex_login::AuthManager;
 use codex_login::CodexAuth;
 use codex_login::read_codex_access_token_from_env;
@@ -1941,7 +1943,16 @@ async fn run_debug_prompt_input_command(
         });
     }
 
-    let prompt_input = codex_core::build_prompt_input(config, input, /*state_db*/ None).await?;
+    let user_instructions_provider = Arc::new(CodexHomeUserInstructionsProvider::new(
+        config.codex_home.clone(),
+    ));
+    let prompt_input = codex_core::build_prompt_input(
+        config,
+        input,
+        /*state_db*/ None,
+        user_instructions_provider,
+    )
+    .await?;
     println!("{}", serde_json::to_string_pretty(&prompt_input)?);
 
     Ok(())