Add one-command GitHub onboarding

[zhou-wang-git]

What changed

• add bugbundle init --github to generate .bugbundle.yml and a GitHub Issue Form
• refuse partial or accidental overwrites unless --force is explicit
• pin generated reporter commands to the running CLI version
• document the one-command adoption flow and keep the copy-paste example aligned
• add core and CLI regression coverage, including YAML-injection protection

Why

The design-partner pilot needs an onboarding path that maintainers can adopt in about a minute without manually copying files or risking existing repository configuration.

Validation

• pnpm check
• pnpm build
• manual black-box smoke test in a temporary repository