feat: wire goal extension tools to the dedicated goal store

[jif-oai]

Why

ext/goal already had the tool specs and contributor wiring for /goal, but the installed tools still depended on a placeholder backend that always errored. That meant the extension could not actually own goal persistence even though the dedicated thread_goals store already exists.

This change wires the extension tools directly to the dedicated goal store so the extension can create, read, and complete goals against real state instead of falling back to host-side placeholders.

What changed

• make install_with_backend(...) require Arc<codex_state::StateRuntime> so goal storage is always available when the extension is installed
• remove the unused no-backend/public backend abstraction from ext/goal and have the tool executors talk directly to StateRuntime
• map thread_goals rows into the existing protocol response shape for get_goal, create_goal, and update_goal
• preserve current thread-list behavior by filling an empty thread preview from the goal objective when a goal is created through the extension path
• add integration coverage for the installed tool surface, including successful goal creation and duplicate-create rejection

Testing

• cargo test -p codex-goal-extension

[jif-oai]

@codex review

[chatgpt-codex-connector]

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