docs: correct the pages publish procedure

`merge --ff-only main` cannot work: the histories diverged some time ago, so
the step fails with `Not possible to fast-forward` and the publish stalls.
Document the normal merge the branch's own history already uses, the stale
local `pages` fast-forward that has to happen first, the take-main-wholesale
conflict resolution, and the tree-equality check that is the real invariant.

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

docs/operations-guide.md

@@ -117,7 +117,8 @@ Use a temporary worktree so the current checkout stays on `main`:
 
 ```bash
 git worktree add /tmp/ai-for-dummies-pages pages
-git -C /tmp/ai-for-dummies-pages merge --ff-only main
+git -C /tmp/ai-for-dummies-pages merge --ff-only origin/pages   # local pages is often stale
+git -C /tmp/ai-for-dummies-pages merge --no-edit main
 git -C /tmp/ai-for-dummies-pages push origin pages
 git worktree remove /tmp/ai-for-dummies-pages
 ```
@@ -125,6 +126,27 @@ git worktree remove /tmp/ai-for-dummies-pages
 The `pages` branch should represent the exact published source. Avoid editing
 it directly and avoid force-pushing it.
 
+The two histories have diverged — `pages` carries merge commits and
+cherry-picked duplicates of `main` commits — so `merge --ff-only main` fails
+with `Not possible to fast-forward`. A normal merge is correct here, and
+matches the `Merge branch 'main' into pages` commits already on the branch.
+The invariant worth checking is the *tree*, not the history:
+
+```bash
+git rev-parse main^{tree} pages^{tree}   # must print the same hash twice
+```
+
+If the merge conflicts (duplicated commits touching the same lines will do
+it), resolve by taking `main` wholesale, since `main` is the source of truth
+for published content:
+
+```bash
+git -C /tmp/ai-for-dummies-pages checkout main -- .
+git -C /tmp/ai-for-dummies-pages add -A
+git -C /tmp/ai-for-dummies-pages diff --cached main --stat   # must be empty
+git -C /tmp/ai-for-dummies-pages commit --no-edit
+```
+
 ### 6. Verify the deployment
 
 ```bash
@@ -491,7 +513,8 @@ git switch main
 git revert BAD_COMMIT
 git push origin main
 git worktree add /tmp/ai-for-dummies-pages pages
-git -C /tmp/ai-for-dummies-pages merge --ff-only main
+git -C /tmp/ai-for-dummies-pages merge --ff-only origin/pages   # local pages is often stale
+git -C /tmp/ai-for-dummies-pages merge --no-edit main
 git -C /tmp/ai-for-dummies-pages push origin pages
 git worktree remove /tmp/ai-for-dummies-pages
 ```