Support relative-path references in `--update`

## Community Note

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### Tell us about your request

Support relative-path references as described by RFC-3986, allowing something similar to `--update=file:relative/path/to/config.yml`.

This would introduce some new ambiguity—is that a URI or a YAML string?—but not a new form of ambiguity. The CLI already decides to treat `file:/not-a-path` as a URI rather than as literal configuration. Maybe the right way forward is to let different options be responsible for URIs (which could then just be paths) and literal configuration.

### Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?

The same as in #93, reference configuration files by relative path. This would be especially nice to have in collaborative projects that don't necessarily live at the same absolute path for different collaborators.

### Are you currently working around this issue?

Yes, with `file://$(pwd)/relative/path/to/config.yml`

Start by tracing how the --update option distinguishes file: URIs from literal YAML configuration, then read the related problem in issue #93. Define the expected behavior for a relative path such as file:relative/path/to/config.yml and verify it against the existing absolute-path workaround before considering the work complete.

Written by the indexing model from the issue text.