fix: detect shared-token misconfiguration and warn

When two review-bot roles share the same Gitea user token (misconfiguration),
log a WARNING identifying which sibling is sharing. The bot continues normally
with its own honest verdict — no escalation, no deadlock. Operators see the
warning in CI logs and can fix the token setup.

Addresses Aaron's review feedback on #28: graceful degradation when someone
doesn't follow the separate-token deployment instructions.

cmd/review-bot/main_test.go

@@ -188,3 +188,68 @@ func TestFindOwnReview(t *testing.T) {
 		})
 	}
 }
+
+func TestWarnSharedToken(t *testing.T) {
+	// warnSharedToken should not panic and should handle edge cases gracefully.
+	// It only logs — we verify it doesn't crash.
+	tests := []struct {
+		name     string
+		reviews  []gitea.Review
+		sentinel string
+	}{
+		{
+			name:     "no reviews",
+			reviews:  nil,
+			sentinel: "<!-- review-bot:sonnet -->",
+		},
+		{
+			name: "no own review yet",
+			reviews: []gitea.Review{
+				{ID: 1, User: struct{ Login string `json:"login"` }{Login: "other"}, Body: "<!-- review-bot:gpt --> body"},
+			},
+			sentinel: "<!-- review-bot:sonnet -->",
+		},
+		{
+			name: "separate users - no warning",
+			reviews: []gitea.Review{
+				{ID: 1, User: struct{ Login string `json:"login"` }{Login: "sonnet-review-bot"}, Body: "<!-- review-bot:sonnet --> body"},
+				{ID: 2, User: struct{ Login string `json:"login"` }{Login: "security-review-bot"}, Body: "<!-- review-bot:security --> body"},
+			},
+			sentinel: "<!-- review-bot:sonnet -->",
+		},
+		{
+			name: "shared token detected",
+			reviews: []gitea.Review{
+				{ID: 1, User: struct{ Login string `json:"login"` }{Login: "sonnet-review-bot"}, Body: "<!-- review-bot:sonnet --> body"},
+				{ID: 2, User: struct{ Login string `json:"login"` }{Login: "sonnet-review-bot"}, Body: "<!-- review-bot:security --> body"},
+			},
+			sentinel: "<!-- review-bot:sonnet -->",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			// Should not panic
+			warnSharedToken(tc.reviews, tc.sentinel)
+		})
+	}
+}
+
+func TestExtractSentinelName(t *testing.T) {
+	tests := []struct {
+		body string
+		want string
+	}{
+		{"<!-- review-bot:sonnet --> rest", "sonnet"},
+		{"<!-- review-bot:security --> rest", "security"},
+		{"no sentinel here", "unknown"},
+		{"<!-- review-bot:gpt-review --> rest", "gpt-review"},
+	}
+
+	for _, tc := range tests {
+		got := extractSentinelName(tc.body)
+		if got != tc.want {
+			t.Errorf("extractSentinelName(%q) = %q, want %q", tc.body, got, tc.want)
+		}
+	}
+}