build, check: report a missing playbook file in check, refuse an unparsed playbook bullet, and reword the drift message

[creedants]

Follow-ups to the playbook resolver and the upstream merge helper.

• scripts/build.py reports override drift as <rel>: upstream changed since this override was written; review or re-port it, then --update-lock. The module docstring says the same.
• scripts/check.py now reports a poteto-mode playbook bullet whose file does not exist. scripts/build.py still fails on that finding before it swaps the tree, and no longer does the lookup itself.
• scripts/catalog.py reads only the Playbooks section, from the line after the level-two heading Playbooks to the next level-two heading.
• A bullet in column one of that section that does not match - **Title.** summary is a finding and fails the check and the build.
• An indented list line in that section is skipped. It is not an entry and not a finding, so a nested note under a playbook does not fail the build.
• AGENTS.md step 5 names the missing playbook file case, and the --merge paragraph names --conflicts <dir>.

A tree built from this branch is byte-identical to one built from main.

Reviewed at 6af6cd5b.