core: make AGENTS.md react to environment changes (#29810)

## Why

With deferred executors, a turn can begin before a remote environment
attaches. AGENTS.md discovery previously ran only during session setup,
so instructions from a later environment never reached the model or the
session instruction sources.

WorldState persistence has now landed, so this uses the durable
model-visible baseline directly instead of carrying a temporary
resume/fork compatibility path.

## What

- Add an `AgentsMdManager` in `SessionServices` to own host
instructions, loaded state, and refresh caching.
- When `DeferredExecutor` is enabled, refresh AGENTS.md when attached
environment selections change and freeze the result in the corresponding
`StepContext`.
- Represent AGENTS.md as a persisted WorldState section for every
session, with bounded initial, replacement, and removal updates.
- Remove duplicate AGENTS.md state and rendering from
`SessionConfiguration` and `TurnContext`.
- Build initial context, per-request updates, and compaction context
from the same step-scoped value.
- On resume and fork, compare current instructions with the restored
WorldState baseline and inject a replacement exactly once when they
differ.

Builds on #29833, #29835, and #29837.

## Tests

- Covers a remote environment becoming ready mid-turn, with AGENTS.md
appearing on the next request exactly once and updating canonical
instruction sources.
- Covers full, unchanged, replaced, and removed AGENTS.md WorldState
rendering.
- Covers changed instructions across cold resume and fork without
duplicate reinjection.
- Covers remote-v2 compaction retaining creation-time instructions in
the live session and cold resume appending one replacement when the
source changed.
- Ran focused `codex-core` AGENTS.md, WorldState, and context-update
test suites.

codex-rs/core/src/session/turn_context.rs

@@ -1,5 +1,4 @@
 use super::*;
-use crate::agents_md::LoadedAgentsMd;
 use crate::environment_selection::TurnEnvironmentSnapshot;
 use crate::shell_snapshot::ShellSnapshotFile;
 use codex_core_skills::HostSkillsSnapshot;
@@ -124,7 +123,6 @@ pub struct TurnContext {
     pub(crate) timezone: Option<String>,
     pub(crate) app_server_client_name: Option<String>,
     pub(crate) developer_instructions: Option<String>,
-    pub(crate) user_instructions: Option<String>,
     pub(crate) collaboration_mode: CollaborationMode,
     pub(crate) multi_agent_version: MultiAgentVersion,
     pub(crate) personality: Option<Personality>,
@@ -273,7 +271,6 @@ impl TurnContext {
             timezone: self.timezone.clone(),
             app_server_client_name: self.app_server_client_name.clone(),
             developer_instructions: self.developer_instructions.clone(),
-            user_instructions: self.user_instructions.clone(),
             collaboration_mode,
             multi_agent_version: self.multi_agent_version,
             personality: self.personality,
@@ -552,10 +549,6 @@ impl Session {
             timezone: Some(timezone),
             app_server_client_name: session_configuration.app_server_client_name.clone(),
             developer_instructions: session_configuration.developer_instructions.clone(),
-            user_instructions: session_configuration
-                .loaded_agents_md
-                .as_ref()
-                .map(LoadedAgentsMd::render),
             collaboration_mode: session_configuration.collaboration_mode.clone(),
             multi_agent_version,
             personality: session_configuration.personality,