PR tables belong in task docs, not story docs

A * PRs table must only appear in task .org files, never in story .org files. Stories track goals and acceptance criteria; tasks track execution, including the PRs that implement them.

Why: Stories and tasks serve different purposes in the doc hierarchy. A story may span multiple tasks and multiple PRs; the task is the right granule to associate with a concrete PR.

How to apply: After gh pr create, open or create the relevant task doc (under the story's folder) and record the PR number there. Do not add a * PRs section to the story doc.