refactor(gen): modernize generated go (any, gofumpt)

[cbrgm]

What

Mechanical Go modernization of the generated code, plus formatting-tool consistency.

• interface{} -> any in the runtime template
• drop the redundant callbacks == nil || len(callbacks) == 0 checks (a nil slice has len 0)
• standardize on gofumpt across the generator, make format, and lint (they disagreed before: generator used gofmt -s, make format used go fmt, lint wanted gofumpt). Generator now formats in-process via go/format, make generate/make format run pinned gofumpt v0.7.0.

Stacked on #279.

Why

Small, safe cleanups that make the generated code read like modern Go and stop the three formatters from fighting. any is an alias for interface{}, so the public API is untouched (apidiff confirms). The generated tree turned out to be already gofumpt-clean, so the reformat produced no churn.

Testing

• make generate -> git diff --exit-code clean and idempotent with pinned gofumpt
• grep -rl 'interface{}' githubevents/ and grep -rl 'callbacks == nil' githubevents/ -> empty
• make test green, make apidiff -> API compatible, golangci-lint 0 issues

Checklist

• Tests added/updated (existing generated suite exercises the changed handlers; no new behavior)
• No breaking changes (any is an alias, checks are behavior-identical, apidiff compatible)
• Readable commit history (2 commits: format, then any/checks)
• AI code review considered and comments resolved