feat(factory): add GitHub issue trigger provider

[cnicolov]

Summary:

• Add bach-github-issue-trigger as a bach.trigger.v1 GitHub Issues provider.
• Map GitHub issues into Factory Work Items with label routing, priority labels, metadata, and updated_at cursors.
• Document provider configuration in README and reference docs.

Verification:

• go run ./cmd/bach run shell/fmt
• go run ./cmd/bach run shell/docs-generate
• go run ./cmd/bach run shell/test
• go run ./cmd/bach run --dry-run shell/lint
• go run ./cmd/bach run shell/lint