core: reuse parent shell snapshot for thread-spawn subagents (#13052)

## Summary
- reuse the parent shell snapshot when spawning/forking/resuming
`SessionSource::SubAgent(SubAgentSource::ThreadSpawn { .. })` sessions
- plumb inherited snapshot through `AgentControl -> ThreadManager ->
Codex::spawn -> SessionConfiguration`
- skip shell snapshot refresh on cwd updates for thread-spawn subagents
so inherited snapshots are not replaced

## Why
- avoids per-subagent shell snapshot creation and cleanup work
- keeps thread-spawn subagents on the parent snapshot path, matching the
intended parent/child snapshot model

## Validation
- `just fmt` (in `codex-rs`)
- `cargo test -p codex-core --no-run`
- `cargo test -p codex-core spawn_agent -- --nocapture`
- `cargo test -p codex-core --test all
suite::agent_jobs::spawn_agents_on_csv_runs_and_exports`

## Notes
- full `cargo test -p codex-core --test all` was left running separately
for broader verification

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/codex_delegate.rs

@@ -62,6 +62,7 @@ pub(crate) async fn run_codex_thread_interactive(
         Vec::new(),
         false,
         None,
+        None,
     )
     .await?;
     let codex = Arc::new(codex);