Support --json flag in rwx lint

[robinaugh]

Summary

• The global --json flag was ignored by rwx lint because it used its own local --output flag
• Now rwx lint --json behaves identically to rwx lint -o json, consistent with other commands like run, sandbox, dispatch, etc.

Manual test plan

# Introduce a lint error (e.g., add an unknown key to a .rwx yaml file)

# Verify --json produces JSON output
rwx lint --json
# Expected: {"Diagnostics":[{"Severity":"error","Message":"...","FilePath":"...","Line":...,"Column":...}],"FileCount":...,"FixedCount":0}

# Verify -o json still works
rwx lint -o json
# Expected: same JSON output as above

# Verify --json overrides -o if both are passed
rwx lint -o oneline --json
# Expected: JSON output (--json takes precedence)

# Verify default output is unchanged without --json
rwx lint
# Expected: multiline format with human-readable diagnostics

# Verify clean files produce empty diagnostics in JSON
rwx lint --json
# Expected: {"Diagnostics":[],"FileCount":...,"FixedCount":0}