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/rules/git-worktrees.md

@@ -36,9 +36,22 @@ writer; everyone else opens an issue in their task report instead of editing:
 | `astro.config.mjs`, `package.json` | `astro-architect`       |
 | `src/content/config.ts`            | `content-i18n-migrator` |
 
+## Never rebase a task branch
+
+Not onto `main`, not onto another task branch, not "just to tidy up". A rebase
+rewrites every commit with a new SHA, so a branch that others branched from — or
+that has already been merged — turns into a duplicate history that no longer
+shares an ancestor with the original. Task 03 did this and flattened task 01's
+merge into four look-alike commits; it is the same divergent-history trap that
+broke the `pages` branch.
+
+To pick up new work from `main`, **merge it in**: `git merge origin/main`. The
+extra merge commit is the price of a history that stays true, and it is cheap.
+
 ## Before you start
 
-1. `git fetch origin && git rebase origin/main` — start from current `main`.
+1. `git fetch origin` — then branch from the ref your task file names as its
+   base. Do not assume that base is `main`.
 2. Read your task file end to end before writing anything.
 3. Confirm your task's dependencies are merged. Task files list them.
 
@@ -46,7 +59,8 @@ writer; everyone else opens an issue in their task report instead of editing:
 
 1. `pnpm run verify` green — without deleting assertions.
 2. The relevant checklist in [`../checklists/`](../checklists/) complete.
-3. `git rebase origin/main` again, resolve conflicts in your worktree.
+3. `git fetch origin && git merge origin/main`, resolve conflicts in your
+   worktree. **Merge — never rebase.**
 4. Task report: what changed, what you verified, **what you did not do**.
 
 ## Cleanup