Adopt PR -- bootstrap branch-context on an existing PR

Populate issue-brief.md and pr-decisions.md for a worktree whose PR already exists and has history.

When to use

• You checked out an existing PR and the branch-context files are empty templates (or absent).
• You are picking up someone else's PR.
• Your own PR predates the branch-context setup and you want to backfill.

Do not use this for fresh work with no PR yet -- write the brief from the linked issue directly.

Startup