## What
- Add a small extension capability for injecting model-visible response
items into the active turn
- Have the goal extension inject hidden goal-context steering when
tool-finish accounting reaches `BudgetLimited`
- Cover the extension backend path with an assertion on the injected
steering item
## Why
PR #23696 persists and emits the budget-limited goal update from
tool-finish accounting, but it leaves the model unaware of that
transition. The existing core runtime steers the model to wrap up in
this case; the extension path should do the same through an explicit
host capability.
## Testing
- `just fmt`
- `cargo test -p codex-goal-extension`
- `cargo test -p codex-extension-api`
## 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`
