A few notes:
- The fallback rule (i.e. if `search_replace` fails twice, use
`write_file` instead) is included.
- I also included instructions to use multiple `search_replace` calls
for moderately large edits with distinct sections. My general
observation has been that often models tend to lean towards using
`write_file` rather than `search_replace` when it's ambiguous, so I
_think_ it should be okay to nudge them towards `search_replace` a
little bit more. Please correct me if I'm wrong about this.
- Gemini pointed out that this can lead to a race condition if two
`search_replace` calls run simultaneously on the same file. I've added
locks to `search_replace` and `write_file` to account for this just in
case.
- Another option would be to extend `search_replace` to account for
multiple changes so they can get batched, but this would be a larger
change.
- I have not changed the basic agent. I can do that if desired.
- I did do some testing to check that models can still manage with the
change of prompt. I haven't noticed any issues.

The following snapshots/fixtures have been updated:
- src/\_\_tests\_\_/\_\_snapshots\_\_/local_agent_prompt.test.ts.snap
-
e2e-tests/snapshots/local_agent_basic.spec.ts_local-agent---dump-request-1.txt
-
e2e-tests/snapshots/local_agent_basic.spec.ts_local-agent---read-then-edit-1.aria.yml
- e2e-tests/snapshots/local_agent_basic.spec.ts_after-edit.txt
-
e2e-tests/snapshots/local_agent_advanced.spec.ts_local-agent---mention-apps-1.txt
-
e2e-tests/snapshots/local_agent_auto.spec.ts_local-agent---auto-model-1.txt
- e2e-tests/fixtures/engine/local-agent/read-then-edit.ts

Which affect the following tests:
- src/\_\_tests\_\_/local_agent_prompt.test.ts
- e2e-tests/local_agent_basic.spec.ts
- e2e-tests/local_agent_auto.spec.ts
- e2e-tests/local_agent_summarize.spec.ts
- e2e-tests/local_agent_advanced.spec.ts

These tests appear to pass.

This PR would also leave a lot of unused code related to `edit_file`,
which might be worth removing (not sure whether to do this).