[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/ext/extension-api/src/lib.rs

@@ -2,6 +2,7 @@ mod capabilities;
 mod contributors;
 mod registry;
 mod state;
+mod user_instructions;
 
 pub use capabilities::AgentSpawnFuture;
 pub use capabilities::AgentSpawner;
@@ -64,3 +65,7 @@ pub use registry::ExtensionRegistryBuilder;
 pub use registry::empty_extension_registry;
 pub use state::ExtensionData;
 pub use state::ExtensionDataInit;
+pub use user_instructions::LoadUserInstructionsFuture;
+pub use user_instructions::LoadedUserInstructions;
+pub use user_instructions::UserInstructions;
+pub use user_instructions::UserInstructionsProvider;

codex-rs/ext/extension-api/src/user_instructions.rs

@@ -0,0 +1,41 @@
+use std::future::Future;
+use std::pin::Pin;
+
+use codex_utils_absolute_path::AbsolutePathBuf;
+
+/// User instructions supplied by the host.
+///
+/// `source` must be an absolute filesystem path because the app-server
+/// `instructionSources` API currently exposes instruction sources as
+/// `AbsolutePathBuf` values.
+// TODO(anp): Replace the absolute path with a more general instruction-source
+// abstraction when non-filesystem providers need first-class attribution.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct UserInstructions {
+    /// Model-visible user instruction text.
+    pub text: String,
+    /// Absolute filesystem path reported through `instructionSources`.
+    pub source: AbsolutePathBuf,
+}
+
+/// Result of loading host-provided user instructions.
+#[derive(Clone, Debug, Default, Eq, PartialEq)]
+pub struct LoadedUserInstructions {
+    /// Loaded instructions, or `None` when the provider has no applicable text.
+    pub instructions: Option<UserInstructions>,
+    /// Recoverable loading problems that should be surfaced during startup.
+    pub warnings: Vec<String>,
+}
+
+/// Future returned by a [`UserInstructionsProvider`].
+pub type LoadUserInstructionsFuture<'a> =
+    Pin<Box<dyn Future<Output = LoadedUserInstructions> + Send + 'a>>;
+
+/// Loads the user instructions that apply when a root thread runtime starts.
+///
+/// Implementations should return any recoverable loading problems as warnings
+/// while still returning usable fallback instructions when available.
+pub trait UserInstructionsProvider: Send + Sync {
+    /// Loads the snapshot to use for a newly created root runtime.
+    fn load_user_instructions(&self) -> LoadUserInstructionsFuture<'_>;
+}