refactor(githubevents): generic dispatch helper

[cbrgm]

What

Collapse the ~500 copy-pasted concurrency loops into one hand-written generic helper.

• new githubevents/dispatch.go with dispatch[T], the single implementation of the errgroup + panic-recovery + first-error-wins loop
• generated handle* methods now call dispatch(...) instead of repeating ~27 lines each
• characterization tests (githubevents/dispatch_test.go) lock the behavior through the public API before the refactor

Net: +722 / -5579 lines. Stacked on #280.

Why

The same errgroup/recover block was generated hundreds of times. One generic helper is far easier to reason about and test, and it only touches unexported internals so the public API is unchanged.

Behavior is preserved exactly, and i was deliberate about it: same errgroup first-error-wins semantics (not errors.Join, so if two callbacks fail only the first surfaces, like before), the exact "recovered from panic: %v" message, and all the guard / wrong-action errors verbatim. The characterization tests (panic recovery, fan-out count, error propagation, empty-action rejection, the Any path) were written to pass on the old code first, then kept passing after the refactor.

Testing

• go test ./githubevents/ -run TestDispatch -v -> 5/5, unchanged before and after the refactor
• make test green (incl. the 100+ generated per-event test files), make apidiff -> API compatible
• grep -rl errgroup githubevents/ -> only dispatch.go + events.go (the bespoke handleError), no events_*.go

Checklist

• Tests added/updated (dispatch_test.go characterization suite)
• No breaking changes (unexported internals only, apidiff compatible)
• Readable commit history (tests first, then the helper)
• AI code review considered and comments resolved