feat: flag repos that stopped receiving pushes

The maintenance criterion was unfalsifiable, so entries went years
without a push before anyone noticed by hand. The updater already
fetches pushedAt; warn when it is older than 90 days, alongside the
existing rename and archival annotations. Archived repos keep their own
warning rather than getting two.

Quantify the criterion in the README and CONTRIBUTING: dropped after 6
months idle, flagged past 3, with removal still a human decision.

Also correct two CONTRIBUTING claims the code contradicts — duplicates
are rejected by CI rather than ignored at run time, and renames do carry
history over once canonicalKeyMigrations has the old key.

github.go

@@ -68,6 +68,12 @@ var httpClient = &http.Client{Timeout: 30 * time.Second}
 // a network call or token.
 var graphqlURL = "https://api.github.com/graphql"
 
+// staleWarnAfter is how long a tracked repo may go without a push before the
+// updater flags it for review. It mirrors the review window in the README's
+// inclusion criteria; dropping an entry stays a human decision, so this only
+// annotates the run.
+const staleWarnAfter = 90 * 24 * time.Hour
+
 // chunkSize is the max aliases per GraphQL request (GitHub node-limit safety margin).
 const chunkSize = 50
 
@@ -111,14 +117,20 @@ func fetchStats(token string, agents []Agent) ([]Stat, error) {
 			lang = node.PrimaryLanguage.Name
 		}
 
-		// Drift detection: surface renames/archival as GitHub Actions run
-		// annotations so a human notices without polling every repo by hand.
+		// Drift detection: surface renames, archival and staleness as GitHub
+		// Actions run annotations so a human notices without polling every
+		// repo by hand.
 		canonicalKey := a.Owner + "/" + a.Repo
 		if node.NameWithOwner != "" && !strings.EqualFold(node.NameWithOwner, canonicalKey) {
 			fmt.Printf("::warning::repo %s renamed to %s — update data/agents.yml and add a canonicalKeyMigrations entry\n", canonicalKey, node.NameWithOwner)
 		}
-		if node.IsArchived {
+		switch {
+		case node.IsArchived:
+			// Archived already implies no further pushes; one warning is enough.
 			fmt.Printf("::warning::repo %s is archived — consider removing or annotating\n", canonicalKey)
+		case !node.PushedAt.IsZero() && timeNow().Sub(node.PushedAt) > staleWarnAfter:
+			days := int(timeNow().Sub(node.PushedAt).Hours() / 24)
+			fmt.Printf("::warning::repo %s has no push in %d days — review against the maintenance criterion\n", canonicalKey, days)
 		}
 
 		stats = append(stats, Stat{