chore: isolate thread goal storage behind GoalStore

[jif-oai]

Why

Thread goal persistence is being prepared for a dedicated storage boundary. Before that split, goal-specific reads, writes, accounting, and cleanup were exposed directly on StateRuntime, so core and app-server callsites stayed coupled to the full runtime instead of a goal-specific store.

This PR introduces that boundary without changing the goal wire API or current persistence behavior. Callers now go through StateRuntime::thread_goals() and the new GoalStore, while GoalStore still uses the existing state DB pool underneath.

What changed

• Added GoalStore in state/src/runtime/goals.rs and exposed it from StateRuntime via thread_goals().
• Moved thread-goal reads, writes, status updates, pause, delete, and usage accounting onto GoalStore.
• Updated core session goal handling, app-server goal RPCs, resume snapshots, and goal tests to use the store boundary.
• Kept thread deletion responsible for cascading goal cleanup by deleting the goal through the store only after a thread row is removed.

Testing

• Existing goal persistence, resume, and accounting tests were updated to exercise the new GoalStore access path.

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

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