feat: generate share summaries with local agents

[xinyao27]

Summary

• make Summary the canonical share concept across CLI, Desktop, Hub, and Web
• publish the raw Session before optionally generating and uploading a local-Agent Summary
• detect and safely invoke installed Claude Code or Codex CLIs without file or terminal tools
• migrate interactive CLI flows to @clack/prompts, with stable non-TTY output
• share prompt and publish logic between CLI and Desktop

Validation

• pnpm build
• pnpm exec vp check
• CLI typecheck + 142 tests
• Desktop typecheck + 14 focused tests
• Backend typecheck + 298 tests
• Web typecheck + 138 tests

E2E was not run, per repository instructions.