[codex] Bind shell snapshots to retained thread environments (#28421)

## Why

Shell snapshots are currently session-scoped even though shell and cwd
are properties of a selected turn environment. That makes snapshot
refresh depend on separate session-cwd plumbing, prevents retained
environments from retaining their snapshot work, and can make snapshot
construction use a different shell than command execution.

This follows #27955 by making the retained thread-environment service
own environment snapshot lifecycles. Session configuration remains the
requested selection state, while `ThreadEnvironments` remains the source
of successfully resolved environments.

## What changed

- Configure the shell-snapshot builder before initial environment
resolution.
- Start each local environment snapshot task when its `TurnEnvironment`
is built and retain that shared task while environment ID and cwd still
match.
- Inherit retained environment snapshots into spawned child threads.
- Carry the selected `TurnEnvironment` through shell runtimes so
snapshot construction and command execution use the same
environment-specific shell and cwd.
- Load project instructions and warm plugins/skills after initial
environment resolution.
- Continue decoding invalid UTF-8 instruction files lossily without
emitting a startup warning.
- Keep requested selections in `SessionConfiguration`; failed or
duplicate resolutions only affect the resolved environment snapshot.

## Validation

- `cargo check -p codex-core --tests`
- `just test -p codex-home instructions` (6 passed)
- Focused environment, instruction, shell-snapshot, and user-shell tests
(84 passed)
- Focused shell-snapshot, user-shell, and unified-exec tests (126
passed; two event-timing tests passed on retry)

codex-rs/core/src/agents_md.rs

@@ -46,7 +46,7 @@ const AGENTS_MD_SEPARATOR: &str = "\n\n--- project-doc ---\n\n";
 /// Loads project AGENTS.md content and combines it with host-provided user
 /// instructions.
 pub(crate) async fn load_project_instructions(
-    config: &mut Config,
+    config: &Config,
     user_instructions: Option<UserInstructions>,
     environments: &TurnEnvironmentSnapshot,
 ) -> Option<LoadedAgentsMd> {
@@ -89,7 +89,7 @@ pub(crate) async fn load_project_instructions(
 /// `Ok(None)`. Unexpected I/O failures bubble up as `Err` so callers can
 /// decide how to handle them.
 async fn read_agents_md(
-    config: &mut Config,
+    config: &Config,
     fs: &dyn ExecutorFileSystem,
     environment_id: &str,
     cwd: &AbsolutePathBuf,
@@ -126,8 +126,6 @@ async fn read_agents_md(
             Err(err) if err.kind() == io::ErrorKind::NotFound => continue,
             Err(err) => return Err(err),
         };
-        warn_invalid_utf8(&p, &data, "Project", &mut config.startup_warnings);
-
         let size = data.len() as u64;
         if size > remaining {
             data.truncate(remaining as usize);
@@ -503,20 +501,6 @@ impl InstructionProvenance {
     }
 }
 
-fn warn_invalid_utf8(
-    path: &AbsolutePathBuf,
-    data: &[u8],
-    source: &str,
-    startup_warnings: &mut Vec<String>,
-) {
-    if let Err(err) = std::str::from_utf8(data) {
-        startup_warnings.push(format!(
-            "{source} AGENTS.md instructions from `{}` contain invalid UTF-8: {err}. Invalid byte sequences were replaced.",
-            path.display()
-        ));
-    }
-}
-
 #[cfg(test)]
 #[path = "agents_md_tests.rs"]
 mod tests;