…[] cache-stable

## Summary

The background skill/memory-review fork constructed a child `AIAgent`
without propagating `enabled_toolsets` / `disabled_toolsets` from the
parent. When the parent narrowed its toolset (via `hermes tools
disable` or `config.yaml`), the fork's default `enabled_toolsets=None`
expanded to "all registered tools" — and the fork's outbound request
body sent a wider `tools[]` array than the parent's main-turn request.

Anthropic's prompt-cache key includes the `tools[]` array byte-for-byte,
so this divergence forked the cache lineage on every nudge and forced a
full prefix rewrite. On a captured ~4 hour Claude-via-Hermes session
this cost roughly 4.3 M cache-write tokens — about half of those
attributable to the per-nudge alternation between the main turn's
narrowed `tools[]` and the review fork's wider `tools[]`.

## Goal

Extend the byte-stability invariant established by PR #17276 (which
fixed `system`) to the `tools[]` slot of the request body, so the
review fork's outbound request hits the parent's warmed Anthropic
prefix cache regardless of how the parent's toolset is configured.

## Implementation

Two-line change in `agent/background_review.py`: pass
`enabled_toolsets=getattr(agent, "enabled_toolsets", None)` and the
matching `disabled_toolsets` kwarg into the `AIAgent(...)` call inside
`_spawn_background_review`. Adds an explanatory block comment that
calls out the cache-key dependency and the relationship to PR #17276.

The post-construction runtime whitelist
(`set_thread_tool_whitelist({memory, skills})`) is untouched — it
still gates which tools the model is allowed to *dispatch*. This
change aligns only what the request body *transmits*, not what the
review is allowed to do, so the safety contract from issue #15204
remains intact.

## Testing

- `tests/run_agent/test_background_review_cache_parity.py`: new
  `test_review_fork_inherits_parent_toolset_config` asserts the
  parent's `enabled_toolsets` and `disabled_toolsets` reach the
  review-fork constructor as kwargs.
- `tests/run_agent/test_background_review_toolset_restriction.py`:
  the existing `test_background_review_does_not_narrow_toolset_schema`
  was inverted (its old "must NOT pass enabled_toolsets" rule was
  built on the assumption that the parent always ran with the
  registry default — wrong in practice when the parent is narrowed).
  Renamed to `test_background_review_matches_parent_toolset_config`
  and updated to assert the parent's value propagates verbatim.
- Verified the new positive test fails without the fix and passes
  with it.
- Full suite for `test_background_review*`:

  ```
  $ python -m pytest tests/run_agent/test_background_review.py \
                     tests/run_agent/test_background_review_summary.py \
                     tests/run_agent/test_background_review_toolset_restriction.py \
                     tests/run_agent/test_background_review_cache_parity.py -q
  18 passed in 1.85s
  ```

## Scope

- `agent/background_review.py`: 2 added kwargs + explanatory comment.
- Two test files: one new positive test, one inverted existing test.
- No production code paths outside the review fork; no schema changes;
  no public-API changes.

Refs: ziliangpeng#1 (root-cause analysis with wire-level
cache-write measurements). Extends PR #17276's `system`-bytes
invariant to the `tools[]` slot.