Extend basectl prompt ergonomics

[codeforester]

Summary

Track follow-up ergonomics for basectl prompt after the initial product-self-review MVP lands.

Candidate Enhancements

• basectl prompt <name> --output <path> to write rendered prompt Markdown.
• basectl prompt <name> --copy for clipboard integration on supported platforms.
• basectl prompt <name> --include-context to append or reference an export-context bundle when appropriate.
• Optional project-scoped prompts once the repo-level contract proves useful.

Acceptance Criteria

• Each enhancement has a narrow contract and focused tests before implementation.
• Base remains a prompt generator/orchestrator, not an AI runtime.
• Any context-including behavior has explicit privacy and size boundaries.

Dependency

Follow-up to the initial basectl prompt product-self-review implementation.