Support labeling through regex substitutions

E.g., a single rule that matches: "- [x] .*?((topic:\W+))" labels the issue with that label "$1" if it exists?

No file, test, or entry point is named in the issue. Start by locating the rule-processing entry point and tracing how regex matches and labels are currently handled; done means a substitution such as "$1" can select an existing label from the captured text, with coverage for the example rule.

Written by the indexing model from the issue text.