fix(format): default ruff to check mode

[nightcityblade]

Fixes #3417

Summary

• Make implicit Ruff formatting default to ruff format --check, matching Black’s read-only behavior.
• Preserve explicit rtk format ruff format write mode and --diff behavior.

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• Manual testing: rtk <command> output inspected
  • N/A: command construction is covered by test_ruff_defaults_to_check_mode; Ruff execution is not needed for this argument-only change.

Important: All PRs must target the develop branch (not master).
See CONTRIBUTING.md for details.

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[HetCreep]

Confirmed — built this branch and drove format_cmd::run() directly with fake ruff.bat/black.bat stand-ins that echo their argv:

ruff project, bare `rtk format`         -> format --check .   (was write mode — fixed)
ruff project, `rtk format --diff`       -> format --diff .    (still read-only)
ruff project, `rtk format format`       -> format             (escape hatch preserved)
black project, bare `rtk format`        -> --check .          (unchanged, no regression)

All four match the intended behavior. Thanks for picking this up.