fix: restore goal accounting after thread resume

[jif-oai]

Why

Goal idle accounting is supposed to survive a thread resume. Previously, the resume hook restored the active goal state inline from the extension lifecycle contributor, which left the runtime handle without a reusable restoration path and made the behavior hard to cover directly. When a thread with an active goal was resumed, goal accounting could lose track of the active idle goal instead of continuing to accrue elapsed time.

What changed

• Moved thread-resume restoration into GoalRuntimeHandle::restore_after_resume() so the runtime owns rehydrating active goal accounting from persisted thread goal state.
• Kept disabled goal runtimes as a no-op and preserved the existing warning path when persisted goal state cannot be loaded.
• Added a backend regression test that seeds an active goal, resumes the thread, waits briefly, and verifies elapsed idle time is reflected on the next external goal mutation.

Testing

• Not run locally; this metadata update only rewrote the PR title/body.

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

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