feat(init): prompt for optional projects during interactive setup (#757)

[ydflow]

Summary

• Prompt for declared projects during interactive init when --project is omitted.
• Support selecting multiple projects; an empty selection remains valid.
• Skip the optional picker in non-interactive runs and print a projects set hint.
• Preserve explicit --project behavior and update English/Chinese usage and design docs.

Closes #757

Verification

• npm run typecheck
• npm run lint
• npm run build
• Targeted init project-selection unit tests: 2 passed
• Targeted non-interactive CLI E2E test: 1 passed

Note: the full init unit test file has 5 Windows path assertion failures that also reproduce on the unmodified upstream base.

[github-actions]

• [P1 blocking] Update the deployed setup skill for the new project prompt. src/init.ts:384 introduces a user decision, but skill-data/setup/references/join-member.md:70 and skill-data/setup/references/setup-admin.md:164 still direct agents to run init without asking which projects to select. In an agent-led interactive setup, this can stall at the unexpected prompt or accept blank and incorrectly save projects: []. This also violates the repository rule requiring behavior changes to update affected skill-data/.

The PR description’s testing record is otherwise sufficient: it includes a representative real-CLI E2E run for the runtime change.

[github-actions]

No findings.

• The earlier [P1 blocking] skill documentation issue is resolved in skill-data/setup/references/join-member.md:70 and skill-data/setup/references/setup-admin.md:166.
• The PR description includes a representative non-interactive real-CLI E2E run, so the runtime-change testing requirement is satisfied.