Add open command workflow

[regenrek]

Summary

• add codefetch open subcommand that copies output to clipboard and opens configurable AI chat
• extract clipboard + browser utilities and cover them with new unit tests
• update help/docs/tests and ensure lint, builds, coverage, and full pnpm test all pass

Testing

• pnpm lint
• pnpm -C packages/sdk build
• pnpm -C packages/cli build
• pnpm test

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/regenrek/codefetch@19

npm i https://pkg.pr.new/regenrek/codefetch/codefetch-sdk@19

commit: 7ef6b2a