[core] Support durable external thread goals

[adamvy-oai]

Overview

This is the fifth change in a five-PR stack that improves reusable Rust APIs for embedders. It adds durable external thread goal APIs and persistence for hosts that manage goals outside rollout-backed threads.

What

• add a goal-store boundary for externally managed thread goals
• preserve goal usage and status accounting across restore
• expose the app-server request surface needed to set, read, and clear durable goals

Why

Some embedders need goals to survive independently of rollout-backed thread persistence. The goal-store boundary lets those hosts persist and restore goals without coupling the app server to a particular transport or collaboration product.

Stack

Each PR is based on the preceding branch:

1. #28459 — custom in-process thread stores
2. #28460 — lossless in-process event delivery
3. #28461 — stored-thread view helpers
4. #28462 — rollout-history turn boundaries
5. #30369 — durable external thread goals (this PR)

Validation

• RUST_MIN_STACK=16777216 cargo test -p codex-app-server external_goal_store_allows_goals_for_rolloutless_threads
• cargo test -p codex-state restore_thread_goal_preserves_external_snapshot_accounting