feat(deepagents): add output_mode parameter to the grep tool (#724)

## Summary
- The grep tool's description mentioned output_mode, but it was never an
actual parameter the tool always returned one hardcoded format.
- Added output_mode (files_with_matches/content/count) as a real schema
parameter, using the existing formatGrepMatches helper.

## Test plan
- Added tests in fs.test.ts for files_with_matches and count modes.
- All tests pass.

Note: default output is now sorted by path with no blank lines between
file groups (minor formatting change, same info).

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Co-authored-by: Hunter Lovell <hunter@hntrl.io>