refactor: scaffold workspace packages

[jlitola]

Summary

• Add Bun workspace scaffolding for future core, MCP, and CLI packages while keeping the root githits package publishable.
• Add private placeholder package manifests and source exports for packages/core-internal, packages/mcp, and packages/cli.
• Add bare package-root TypeScript aliases and document private-source bundling/package-boundary lessons.
• Pin @inquirer/prompts and @inquirer/core to the validated pair to preserve ExitPromptError instanceof behavior.

Verification

• Code reviewer: no findings.
• bun install --frozen-lockfile
• bun run typecheck
• bun test
• bun run lint
• bun run format:check
• git diff --check
• bun run build
• npm pack --dry-run

Notes

• No runtime source moved in this PR.
• Root package remains publishable.
• docs/plans/ remains local and is not part of this PR.