fix(requesting-code-review): anchor the multi-commit BASE_SHA alternative to the merge base

The '# or origin/main' alternative fed a moving ref into the reviewer's
two-dot diff: once origin/main advances past the branch point, main's new
files appear as phantom deletions the reviewer can't distinguish from real
ones. Reproduced during triage (2026-08-12): a scratch repo with main
advanced one commit shows 'main-new.txt | 1 -' in the branch's diff.
git merge-base origin/main HEAD anchors the range to the branch point,
matching how sdd's review-package already computes BASE.

Reported in #2118 (wan-huiyan). Fixes #2118.

skills/requesting-code-review/SKILL.md

@@ -25,7 +25,7 @@ Dispatch a code reviewer subagent to catch issues before they cascade. The revie
 
 **1. Get git SHAs:**
 ```bash
-BASE_SHA=$(git rev-parse HEAD~1)  # or origin/main
+BASE_SHA=$(git rev-parse HEAD~1)  # or: git merge-base origin/main HEAD
 HEAD_SHA=$(git rev-parse HEAD)
 ```