Allow schema check without specifying a task

[maddenp-cu]

Currently:

$ wxvx -c config.yaml -k
[2025-08-21T14:12:49]     INFO Available tasks:
[2025-08-21T14:12:49]     INFO   grids
[2025-08-21T14:12:49]     INFO   grids_baseline
[2025-08-21T14:12:49]     INFO   grids_forecast
[2025-08-21T14:12:49]     INFO   plots
[2025-08-21T14:12:49]     INFO   stats

because wxvx expects -t / --task to be specified, but this is irrelevant for schema-checking the config. Make -t optional when -k is specified.

If there's an example in README.md (there may not be), update it to omit -t.