docs: forbid rebasing task branches, and record phase 0 as merged

Task 03 rebased its branch onto task 01 and flattened the merge into four
duplicate commits — but the rule file was *telling* it to: "Before you start:
git rebase origin/main". Replaced with merge-based guidance and an explicit
prohibition, since a rewritten task branch is the same divergent-history trap
that broke the pages branch.

Also corrects gates.md tier 3, which promised screenshot comparison in CI that
is not wired in and cannot be until visual-regression.mjs grows a compare mode.

HANDOVER.md now reflects 01-04 merged, pnpm, and carries forward the review
findings that were noted but deliberately not fixed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.agents/checklists/before-merge.md

@@ -1,6 +1,7 @@
 # Checklist: before you merge a task branch
 
-- [ ] Rebased on current `origin/main`, conflicts resolved in the worktree
+- [ ] `origin/main` **merged in** (never rebased), conflicts resolved in the
+      worktree
 - [ ] `pnpm run verify` and `node scripts/audit-ui.mjs` both green
 - [ ] **Assertion count in `verify.mjs` did not fall**
       (`grep -c 'throw new Error' scripts/verify.mjs`)