docs(wave): rule 9 gates every merge of main and scopes stacked PRs

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

.agents/skills/wave/agent-rules.md

@@ -102,17 +102,38 @@ not in a second copy.
    where each came from). Close the old branch rather than force-pushing it:
    `AGENTS.md`, "Shipping a wave".
 9. **Merge main as soon as you can, and keep merging it.** The user:
-   "Agents should merge main asap" and, asked the point, "Reduces conflicts"
-   (2026-10-09). `git fetch gh`, then merge the new `gh/main` SHA (rule 7's
-   re-pin, done on purpose) at the start of a task, before every push, and
-   every time you resume. Main only gets further from your base, and a
-   conflict met early is small. Never `origin/main`, never rebase, never
-   squash (rule 8). Commit the merge through `scripts/agent-git`.
+   "Agents should merge main asap" and, when asked why, "Reduces conflicts"
+   (2026-10-09 UTC; bd memory `agents-merge-main-asap`, written by the
+   orchestrator; PR #335). Those two quotations are the user's words. The
+   timing below is the orchestrator's operationalisation of them.
+   - **When.** `git fetch gh`, then merge the new `gh/main` SHA (rule 7's
+     re-pin, done on purpose) at the start of a task, before the first review
+     request, before every push to a PR under review, and every time you
+     resume. Main only gets further from your base, and a conflict met early
+     is small. Never `origin/main`, never rebase, never squash (rule 8).
+     Commit the merge through `scripts/agent-git`.
+   - **After every merge of main, clean or not**, re-run the gates (see
+     "Gates") on the merged tree, not on each side, and diff the test names
+     against your pre-merge HEAD to see that none was lost. A clean merge is
+     the dangerous case: it can delete a test module or break a shared test
+     file with no conflict marker.
+   - **Re-measure your baseline** after a mid-task merge, and report it
+     against the merged SHA, so main's changes are not read as yours.
    - **Resolve a conflict by hand.** For each conflicted file, diff the
      result against both parents and the merge base, so a side's
      non-conflicting changes are not lost to a whole-file `--ours` or
-     `--theirs`. Check that no test name was lost. Then re-run the gates
-     (see "Gates") on the merged tree, not on each side.
+     `--theirs`.
+   - **A stacked PR merges its base branch, not `gh/main`.** GitHub diffs it
+     against that base, so merging a `gh/main` ahead of the base puts main's
+     commits in its diff (the #145/#148 symptom, which rule 3's numstat
+     cannot see). The orchestrator retargets it to main once the base lands.
+   - **The exception is narrow.** A durability push of WIP (see "Commit
+     early") need not wait for a merge of main that conflicts. If you cannot
+     resolve the conflict yet, push the WIP to a separate wip branch, never
+     to the PR.
+   - **Each merge-and-push moves the head SHA** an in-flight review is aimed
+     at. Reviewers re-fetch the tip before posting, but do not merge main
+     only to push a reply.
    - The mechanics are `AGENTS.md`, "Shipping a wave" (merge `gh/main`, never
      `origin/main`) and "Merging many PRs at once" (diff each parent; two
      clean changes to one test file can still fail together). Not restated here.