feat(aif-user-test): user testing skill with session guide and feedback record

[natashaannn]

Closes #113
Related to #102

Summary

• Adds skills/aif-user-test — a skill that sets up a structured user testing session for a design branch produced by aif-design-issue.
• Step 2 outputs an interviewee recommendation table (bare minimum vs ideal vs avoid) before any tunnel setup, so the facilitator knows who to line up.
• Step 3 generates a filled session guide (context script, tasks framed as user goals, facilitation rules, per-task observation prompts) and a blank feedback record with scenario names pre-filled.
• Step 4 starts localtunnel by default (no account needed) or ngrok if available, and prints the URL with reminders about keeping the tunnel alive.
• Step 5 hands off to the facilitator — the agent is not involved in the session itself.
• Adds reference/user-testing-guide.md and reference/feedback-record-template.md as the base templates the skill fills in.
• The feedback record's summary section distinguishes blocking changes (for aif-apply-feedback) from deferred changes (for follow-up issues).

Test plan

• Run aif-user-test with a PR number from the aif-design-issue workflow — confirm Step 2 outputs the interviewee table before any tunnel command
• Confirm the session guide is generated with the scenarios from the PR's user testing table pre-filled as tasks
• Confirm the feedback record has the same scenario names pre-filled
• Confirm Step 4 runs npx localtunnel --port <port> and prints the URL

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