[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/app-server/src/message_processor.rs

@@ -72,6 +72,7 @@ use codex_core::config::Config;
 use codex_exec_server::EnvironmentManager;
 use codex_feedback::CodexFeedback;
 use codex_goal_extension::GoalService;
+use codex_home::CodexHomeUserInstructionsProvider;
 use codex_login::AuthManager;
 use codex_login::auth::ExternalAuth;
 use codex_login::auth::ExternalAuthRefreshContext;
@@ -339,6 +340,9 @@ impl MessageProcessor {
                         thread_store: Arc::clone(&thread_store),
                     },
                 ),
+                Arc::new(CodexHomeUserInstructionsProvider::new(
+                    config.codex_home.clone(),
+                )),
                 Some(analytics_events_client.clone()),
                 Arc::clone(&thread_store),
                 state_db.clone(),