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/agents_md.rs

@@ -16,7 +16,6 @@
 //! 3.  We do **not** walk past the project root.
 
 use crate::config::Config;
-use crate::context::ContextualUserFragment;
 use crate::context::UserInstructions as ContextUserInstructions;
 use crate::environment_selection::TurnEnvironmentSnapshot;
 use codex_config::ConfigLayerSource;
@@ -378,8 +377,7 @@ impl LoadedAgentsMd {
         output
     }
 
-    /// Returns the complete model-visible contextual user fragment.
-    pub(crate) fn render(&self) -> String {
+    pub(crate) fn contextual_user_fragment(&self) -> ContextUserInstructions {
         // One contributing project environment retains the legacy cwd wrapper. With two or more,
         // the body labels every contributing environment itself, so the outer cwd is omitted.
         let directory = if self.has_multiple_project_environments() {
@@ -392,12 +390,6 @@ impl LoadedAgentsMd {
             directory,
             text: self.text(),
         }
-        .render()
-    }
-
-    /// Returns the host-provided user instructions.
-    pub(crate) fn user_instructions(&self) -> Option<&UserInstructions> {
-        self.user_instructions.as_ref()
     }
 
     /// Returns the AGENTS.md files that supplied instruction entries.