Add configurable label normalization and event filtering

[darthfork]

Parent issue: #34

Summary

Raw label values such as branch names, runner names, and workflow names are often too granular for efficient Prometheus storage. Add normalization and filtering so operators can keep useful aggregates while bounding series growth.

Ideas

• Branch normalization into classes such as default/release/feature.
• Runner normalization into hosted/self-hosted or configured groups.
• Repo allowlist/denylist.
• Branch and workflow regex filters.

Acceptance criteria

• Operators can reduce label diversity without patching code.
• Documentation explains recommended production configurations.