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/codex_thread.rs

@@ -466,9 +466,15 @@ impl CodexThread {
 
         let turn_context = self.codex.session.new_default_turn().await;
         if self.codex.session.reference_context_item().await.is_none() {
+            // This history-only API runs without run_turn, so it owns its initial step.
+            let step_context = self
+                .codex
+                .session
+                .capture_step_context(Arc::clone(&turn_context))
+                .await;
             self.codex
                 .session
-                .record_context_updates_and_set_reference_context_item(turn_context.as_ref())
+                .record_context_updates_and_set_reference_context_item(step_context.as_ref())
                 .await;
         }
         self.codex