README: list the CATSTACK_DRAFTER_CORE flag

[EdbertChan]

Summary

The readme's settings table now lists the new setting that turns the outside PR rule package back on.

Without this row, a reader of the table would not know the PR rules are off by default or how to turn them on.

Review Claim

The readme settings table lists the new PR rule setting and what it does.

Review Lane

docs

Review Unit

docs

Safety Invariant

Text-only change to one table row; no code, hook, or install path reads it.

Slice Rationale

Docs are their own review unit here, so this row sits on top of the PR that adds the setting.

Non-goals

• Does not change any script or test.

Test Plan

Test Plan

• python3 engine/skills/make-pr/scripts/preflight.py --base <parent branch> --body-file <this body>
• Row reads: CATSTACK_DRAFTER_CORE=1 | env and files | the draft-pr scripts run the drafter-core PR rules; off, they print UNCHECKED and skip them

Revert Plan

Revert Plan

• Safe to revert? Yes
• Revert command: git revert <sha>
• Post-revert steps: None
• Data migration? No

🤖 Generated with Claude Code

Depends-On: #742

[EdbertChan]

This pull request is part of a Mergify stack:

#	Pull Request	Link
1	Turn drafter-core PR rules off unless CATSTACK_DRAFTER_CORE=1	#742
2	README: list the CATSTACK_DRAFTER_CORE flag	#743	👈