docs: document Worktrunk pre-merge gates

[renezander030]

What changed

Documents a blocking Worktrunk pre-merge hook that runs Skillgate in the source worktree after rebase. The command pins Skillgate 0.11.0 and passes the merge target as the base for diff-aware gates.

Why

Worktrunk users can check each agent worktree before merging it into the target branch. Existing Skillgate policy discovery already supports linked worktrees, so the integration needs only one README paragraph and a changelog entry.

How I verified it

• Worktrunk 0.80.0 with published @reneza/skillgate@0.11.0: main and two real linked worktrees used their own policy from both root and nested directories. A failing gate blocked the merge and preserved the target HEAD; passing and repaired worktrees merged successfully, including a rebase.
• npm run build passed.
• npm run test:coverage passed on Node 22.22.0.
• node dist/src/cli.js check passed all four self-gates.
• git diff --check passed.

Checklist

• Verified the documented command against real Worktrunk worktrees; existing behavior required no new tests.
• npm run test:coverage passes locally.
• Documented the integration in the README; gate behavior is unchanged.
• Added an Unreleased changelog entry.
• Evaluator code and dependencies are unchanged.