Add pull request preparation skill

[QuentinTorg]

Intent

Add a narrowly triggered skill that creates structured draft pull requests and supports human-authorized finalization after an independent current-head review.

Delivered change

• Adds the preparing-pull-requests skill with separate draft and finalization modes.
• Preserves repository pull-request templates and scales descriptions to the change.
• Adds safe, idempotent GitHub operation guidance and a concise default template.
• Adds trigger and behavioral evaluation scenarios plus the skill specification.

Verification

• Checked relative resource links and skill metadata.
• Checked Markdown whitespace and staged diff integrity.
• No automated agent evaluation harness was run; the evaluation scenarios are currently a test plan.

Risk and review guidance

This is a draft for independent review. Focus on trigger precision, shared-state authorization boundaries, intent preservation during finalization, and whether the progressive-disclosure split is appropriately concise.