Add ThreadManager sample crate

[pakrym-oai]

Summary:

• Add codex-thread-manager-sample, a one-shot binary that starts a ThreadManager thread, submits a prompt, and prints the final assistant output.
• Pass ThreadStore into ThreadManager::new and expose thread_store_from_config for existing callsites.
• Build the sample Config directly with only --model and prompt inputs.

Verification:

• just fmt
• cargo check -p codex-thread-manager-sample -p codex-app-server -p codex-mcp-server
• git diff --check

Tests: Not run per request.

[chatgpt-codex-connector]

💡 Codex Review

codex/codex-rs/app-server/src/codex_message_processor.rs

Lines 4903 to 4904 in 6c6ec37

	.read_stored_thread_for_new_fork(
	fork_thread_store.as_ref(),

Read forked thread from the same persistence backend

fork_thread_store is derived from the fork request config, but fork_thread_from_history now persists via ThreadManager’s startup-time store. If experimental_thread_store differs at runtime, this read targets a different backend and can fail with thread-not-found right after a successful fork creation.

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