This re-introduces the `save_file` and `restore_file_from_disk` agent
tools that were reverted in #44949.

I pushed that original PR without trying it just to get the build off my
machine, but I had missed a step: the tools weren't added to the default
profile settings in `default.json`, so they were never enabled even
though the code was present.

## Changes

- Add `save_file` and `restore_file_from_disk` to the "write" profile in
`default.json`
- Add `Thread::has_tool()` method to check tool availability at runtime
- Make `edit_file_tool`'s dirty buffer error message conditional on
whether `save_file`/`restore_file_from_disk` tools are available (so the
agent gets appropriate guidance based on what tools it actually has)
- Update test to match new conditional error message behavior

Release Notes:

- Added `save_file` and `restore_file_from_disk` agent tools to handle
dirty buffers when editing files