feat: alternative prioritisation methods (WSJF, Kano, RICE)

[adamdaw]

Add WSJF, Kano, and RICE as alternative prioritisation methods alongside MoSCoW.

Context

MoSCoW is the current default. Different teams and project contexts benefit from different prioritisation approaches — WSJF for flow optimisation, Kano for customer satisfaction, RICE for numeric scoring.

Acceptance Criteria

• /req-02 supports selecting prioritisation method
• gather-requirements.sh supports --prioritisation flag
• Templates include alternative table formats
• Documentation with method selection guidance

Reference: docs/ENHANCEMENTS.md