PR dashboard: skip merge-from-base commits in commit summary context

The is_substantive() filter already excludes "Update branch" merge commits from the latest-event determination, but the LLM-rendered context still surfaced them in the "Last N commits" listing and used their timestamp for "Last commit pushed". That made PRs where the only recent author activity was clicking "Update branch" (e.g. open-telemetry#18090) look like the author had just acted, pushing them into the approver bucket.

Author: trask

.github/scripts/pull-request-dashboard.py

@@ -434,14 +434,25 @@ def is_substantive(e: dict[str, Any]) -> bool:
     # context. Full commit messages and diffs travel via timeline events.
     commit_rows = []
     for c in recent_commits:
-        sha = (c.get("sha") or "")[:7]
+        sha_full = c.get("sha") or ""
+        sha = sha_full[:7]
         msg = (c.get("commit") or {}).get("message", "").splitlines()[0] if c.get("commit") else ""
         a = c.get("author") or {}
         commit_login = a.get("login") or ((c.get("commit") or {}).get("author") or {}).get("name") or "?"
         commit_date = ((c.get("commit") or {}).get("author") or {}).get("date") or ""
-        commit_rows.append({"sha": sha, "msg": msg, "author": commit_login, "date": commit_date})
+        commit_rows.append({
+            "sha": sha,
+            "msg": msg,
+            "author": commit_login,
+            "date": commit_date,
+            "is_merge": sha_full in merge_shas,
+        })
 
-    last_commit_date = parse_ts(commit_rows[-1]["date"]) if commit_rows else None
+    # "Last commit pushed" is meant to surface real author activity. Merge
+    # commits from "Update branch" don't qualify, so skip them when picking
+    # the timestamp.
+    non_merge_rows = [r for r in commit_rows if not r["is_merge"]]
+    last_commit_date = parse_ts(non_merge_rows[-1]["date"]) if non_merge_rows else None
 
     # Checks summary.
     failing = [c for c in checks if (c.get("state") or "").upper() in ("FAILURE", "ERROR")]
@@ -543,7 +554,8 @@ def render_context(ctx: dict[str, Any]) -> str:
     # Commits
     lines.append(f"Last {len(ctx['commits'])} commits (oldest first):")
     for c in ctx["commits"]:
-        lines.append(f"  {c['sha']} {c['date'][:10]} @{c['author']}: {truncate(c['msg'], 120)}")
+        suffix = " [merge from base — not a substantive change]" if c.get("is_merge") else ""
+        lines.append(f"  {c['sha']} {c['date'][:10]} @{c['author']}: {truncate(c['msg'], 120)}{suffix}")
     lines.append("")
 
     # Last substantive event highlight