Add support for multiple named workflow configurations

[Neokil]

Problem

AutoPR supports only a single workflow configuration. This forces users to choose between a full investigation-to-PR pipeline and nothing — there's no way to run a lighter "just refine this ticket" workflow or a custom sequence for a specific use case. Different tasks warrant different workflows and there's currently no way to express that.

Solution

1. Extend the workflow config format to support a named list of workflows (e.g. workflows: [{name: full, ...}, {name: refine-only, ...}]).
2. Add a default_workflow config key.
3. Update the CLI (auto-pr run) to accept an optional --workflow <name> flag.
4. Update the web UI to allow selecting a workflow when starting a ticket run.
5. Store the selected workflow name on the run record so the correct config is used throughout.