Let docket fmt convert a recipe between YAML and JSON5

[josegonzalez]

Description of feature

docket fmt canonically formats a recipe, but it can only ever write back the format it read. There is no way to turn a YAML recipe into its JSON5 twin, or the reverse.

--tasks-format looks like it might do this, but it is a reader override, not a converter. It states what the bytes on disk already are, for when the extension is absent or misleading, or when a flow-style YAML recipe would sniff as JSON5 because it opens with [. TestFmtTasksFormatOverridesFileExtension reads clearly here: it writes a JSON5 body into recipe.yml and passes --tasks-format json5 so the JSON5 formatter is selected. The content was already JSON5. Pointing the same flag at a genuinely-YAML file hands block-style YAML to the JSON5 parser and errors out.

Converting today means going out to another tool and back, which loses the comments both formatters otherwise preserve.

#410 added --format to init and export to state the format of what those commands write, as the output-side counterpart to --tasks-format. fmt was deliberately left out of that change because conversion raises questions the init/export case does not, and they are worth settling before any flag is added:

• Does it rewrite in place under the existing name, leaving tasks.yml holding JSON5? That is a file whose extension lies about its contents, and a later docket validate --tasks tasks.yml picks its parser from the extension and fails.
• Does it rename tasks.yml to tasks.json? That makes fmt a command that deletes files, which is a large change in what the command is.
• Does it require an explicit --output, making conversion a single-file operation? fmt currently takes many paths and expands directories.
• What happens with --check and --diff? A conversion is always a diff, so --check would report every file as unformatted, which is not what the CI lint job using it means.
• Is docket fmt --format json5 - just a pipe-friendly converter, and is that the whole feature people actually want?

The stdin form may be the useful 90% and the safe place to start: cat tasks.yml | docket fmt --format json5 - writes the converted recipe to stdout, touches nothing on disk, and sidesteps every naming question above.