chore: normalize Phase → Step terminology across all 13 skills

[felipecabargas]

Summary

• 9 of 13 skills used "Step"; 4 used "Phase" — inconsistent with no semantic reason for the difference
• This PR normalises the 3 skills on main that used "Phase": competitive-analysis, prioritize, write-product-strategy
• PR feat(write-fr): Step 4 Verification Gate — five-dimension AC scoring after assembly #20 (write-feature-request) carries the same normalization for its file as a separate commit on that branch
• Pure rename — no behavioural changes, no logic modified

Files changed

File	Before	After
skills/competitive-analysis/SKILL.md	Phase 0–3	Step 0–3
skills/prioritize/SKILL.md	Phase 0–3	Step 0–3
skills/write-product-strategy/SKILL.md	Phase 1–3	Step 1–3

Test plan

• Run /gambit:competitive-analysis — confirm behaviour unchanged
• Run /gambit:prioritize — confirm behaviour unchanged
• Run /gambit:write-product-strategy — confirm behaviour unchanged
• Grep for "Phase" across all skills — expect zero results

🤖 Generated with Claude Code