Story: Track PRs per task and surface them in compass where

Every task document carries a * PRs table listing the pull requests raised for that task (number and title). When a PR is created, it is recorded there immediately. compass where --prs (part of the Locate pillar) shows the same data live — fetching open and closed PR status via gh — so a session can orient itself without manually checking GitHub. Story and task UUIDs are embedded in the PR body on creation for forward traceability.

• v2_doc_task.org.mustache has a * PRs section with an empty | PR | Title | table.
• how_do_i_create_a_pr.org instructs the author to record the PR number and title in the task's * PRs table immediately after raising, and to include the story and task :ID: UUIDs in the PR body.
• compass where lists the current version, sprint, and all STARTED stories and tasks with their file paths and UUIDs.
• compass where --prs additionally reads each task's * PRs table and fetches live status for all listed PRs (open and closed) via gh pr view.
• Output is available in pretty (human) and json (tooling) formats.

• PRs table on tasks only, not stories. Stories span many tasks and many PRs; the task is the unit that owns a PR. Aggregation can happen at read time (compass walks task files).
• No status column in the PRs table. Maintaining it would require manual updates after each PR state change. compass fetches live status from gh instead.
• Story + task UUID in PR body, not title. Titles are read by humans at a glance; UUIDs belong in the body where tooling can parse them.
• compass where reads the filesystem directly. The cached FTS index is for search; for where, walking doc/agile/versions/ and parsing #+todo: + the Status table is fast enough and always fresh.

• Retroactive: existing tasks and PRs are not back-filled.
• Closed PR bodies are not parsed for UUID linkage.
• compass where does not mutate any state.